This greatly simplifies the code and as a bonus also shaves off
about 300ms when stripping mscorlib.dll.

When creating a bundle on Windows to run on Android, macOS/iOS or Linux we need
to use the target system's path separator when storing satellite assemblies in
the bundle instead of the host system's one. Failing to do so will embed (on
Windows) the satellite assemblies with paths like

   fr_FR\myassembly.resources.dll

while at runtime Mono will try to look up the assembly by

   fr_FR/myassembly.resources.dll

and fail to find it, obviously. The `--style` parameter is now consulted to see
what path separator character should be used.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=55603

* Improve error messages to help users diagnose problems when using the custom mode

* Update mkbundle.cs

Most of the removed files weren't touched in the last decade
and are unused so it's better to remove them to avoid confusion
about what they do.

I also went over and removed the most outdated info from readme
files, we probably want to do a more thorough overhaul in the future.

Follow-up to https://github.com/mono/mono/pull/4899. I audited
all the places in BCL where we made use of Cecil's AssemblyDefinition
and made sure we're properly disposing them and not keeping files
unnecessarily open.

I used InMemory=true in cases where tracking the lifetime would
be complicated. There are also some tests in mcs/tests which I
didn't bother to fix.

Context: https://bugzilla.xamarin.com/show_bug.cgi?id=56275

Recent Cecil 0.10 changed to not reading the assembly in memory but
instead reads directly from the underlying stream [0]. This however
means that the file handle is not freed until the AssemblyDefinition
is disposed.

Xamarin.Android is using the pdb2mdb source as part of
Xamarin.Android.Build.Tasks.dll and hit an issue where the file
would be locked inside of VS.

[0] http://cecil.pe/post/149243207656/mono-cecil-010-beta-1
(cherry picked from commit 5077205a)

* [mcs] Rework %-recursive target to explicitely list subdirectories

* [mcs] Rename PLATFORM to BUILD_PLATFORM

* [mcs] Add platform specific files

This is to support adding corefx platform specific files. To do that, we now support `mcs/class/corlib/linux_net_4_x_corlib.dll.sources` for example, on top of `mcs/class/corlib/net_4_x_corlib.dll.sources` and `mcs/class/corlib/corlib.dll.sources`.

The 3 platforms that are supported are: `linux`, `darwin` and `win32`.

* [mcs] Build cross-platform bcl in separate folder

Similar to 9635b9ce we
can now break the dependency on WinForms from resx2sr.

This allows us to add it to the "build" profile
and clean up the Makefile.

They were either replaced by the autogenerated csproj files
or just completely broken for years.

monodroid used to build its own version of mkbundle since it
requires slightly different code than the desktop. This
logic was protected in a #if block. Now that monodroid is
trying to make use of the shipped mkbundle rather than use
its own we need to turn this logic on at runtime.

A new parameter has been added

	--bundled-header

When this option is passed, the code which skips including

	mono/metadata/mono-config.h

will be skipped in favour of defining the required structures
and entry points directly.

[cil-stripper] Mark stripped methods as noinline to prevent the JIT from trying to inline them. Fixes #55041. (#4732)

resgen.exe used to load WinForms via reflection which is a pretty
large dependency. Instead we can just embed the few types that
are used for reading/writing resx files directly into the app.

Add System.Drawing.dll to the "build" profile so we can put
resgen there as well and allow us to no longer rely on system
resgen for mobile profiles like monotouch/monodroid.

It confuses the build on Linux otherwise.

This is just confusing now that we have the real thing.

This bumps the reference assemblies submodule so we can install
the new assemblies which are built from source.

This adds a command line argument preprocessor that splits command line
arguments into two groups, delimited by the "standard" `--` "stop
processing" argument, or by the `-s` argument which can be used with
Unix shebang syntax (where the path of the file being executed will
be appended to the command line specified on the shebang line).

- anything before `--` or `-s` will be passed to mcs/csharp as usual
- anything after `--` or `-s` will _not_ be processed by mcs/csharp,
  and will be made available in an `Args` builtin, like `csi`

This _finally_ (after almost 10 years) allows `csharp` to be used for
proper script authoring!

Document the new arguments in the man page. Also remove references to
'gsharp' from the man page.

Wildcards aren't supported in EXTRA_DISTFILES and it wouldn't work
anyway since we need to enclose the paths with spaces in quotes.

When storing satellite assemblies, mkbundle prefixes their names with a
directory name derived from the language/locale of the assembly. It uses the
platform's default directory separator character which on Windows defaults to
'\' and that causes problems when building such a bundle on Windows since \ is
an escape sequence prefix inside strings and not escaping it with another \
leads to gcc errors when building the genrated source.

This commit fixes the problem by quoting the directory separator character
properly on Windows.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=52845