prefer using visibility=hidden for __libc internal data, rather than
an accessor function, if the compiler has visibility.

optimize with -O3 for PIC targets (shared library). without heavy
inlining, reloading the GOT register in small functions kills
performance. 20-30% size increase for a single libc.so is not a big
deal, compared to comparaible size increase in every static binaries.

use -Bsymbolic-functions, not -Bsymbolic. global variables are subject
to COPY relocations, and thus binding their addresses in the library
at link time will cause library functions to read the wrong (original)
copies instead of the copies made in the main program's bss section.

add entry point, _start, for dynamic linker.

this only made the function unnecessarily slow on systems with
unaligned access, but would of course crash on systems that can't do
unaligned accesses (none of which have ports yet).

prior to this change, a large portion of libc was unusable prior to
relocation by the dynamic linker, due to dependence on the global data
in the __libc structure and the need to obtain its address through the
GOT. with this patch, the accessor function __libc_loc is now able to
obtain the address of __libc via PC-relative addressing without using
the GOT. this means the majority of libc functionality is now
accessible right away.

naturally, the above statements all depend on having an architecture
where PC-relative addressing and jumps/calls are feasible, and a
compiler that generates the appropriate code.

this change is in preparation for upcoming PIC/shared library support.
the intent is to avoid going through the GOT, mainly so that dprintf
is operable immediately, prior to processing of relocations. having
dprintf accessible from the dynamic linker will make writing and
debugging the dynamic linker much easier.

this change is made with some reluctance, but i think it's for the
best. correct programs must handle either behavior, so there is little
advantage to having malloc(0) return NULL. and i managed to actually
make the malloc code slightly smaller with this change.

do not allow allocations that overflow ptrdiff_t; fix some overflow
checks that were not quite right but didn't matter due to address
layout implementation.

this is needed in the long term for ABI compatibility anyway, and in
the immediate, it helps with building broken programs like GNU screen
that try to prototype the functions themselves rather than using the
header.

after re-reading 2.2.2 of POSIX 2008, all of these are in the correct
reserved namespaces and do not need protection.

this is a nonstandard header used only by backwards programs, but for
some reason it's extremely popular. the recent namespace cleanup fixes
broke it, because PATH_MAX and NAME_MAX will not be defined unless an
approriate feature test macro has been defined. moreover, it's too
late to just #define _GNU_SOURCE in param.h, since limits.h may have
already been included.

let's just hard-code standard values and be done with it.

note that this library itself is built with -ffreestanding so sincos.c
should not be miscompiled even if the gcc used to compile musl has
this bug.

the existence of a (kernelspace) thread must never have observable
effects after the thread count is decremented. if signals are not
blocked, it could end up handling the signal for rsyscall and
contributing towards the count of threads which have changed ids,
causing a thread to be missed. this could lead to one thread retaining
unwanted privilege level.

this change may also address other subtle race conditions in
application code that uses signals.

this code was wrongly disabled because the old version was trying to
be too clever and didn't work. replaced it with a simple version for
now.

the historic mktemp is supposed to blank the template string on
failure, rather than returning 0. just zero the first character so
that mkstemp and mkdtemp can still retry with O(1) space requirement.