* They didn't work before, because the location of the tests caused the
  'sysroot' option to crate lookup to be wrong for finding the correct stage's
  core/std libraries. This moves the compiled tests from the $host/test
  directory into a $host/$stage/test directory. This means that the sysroot will
  be correct and the core/std libraries can actually be found
* The LLVM bindings apparently aren't threadsafe, so we can't run multiple tests
  in parallel.

(I wonder if there's a better way to write this in the rule itself;
i.e. something like `$$(dirname $$@)`.  But for now this will do.)

This is an attempt to address Issue #3326 by adding [*order-only*][1]
prerequsites of each build product on the directory where it is to go.
It is important that the prerequisites be order-only, since the
timestamp on a parent directory is not relevant to whether a product
is out of date; the parent directory merely needs to exist.

(This use case of generating target directories was provided as an
[example][2] of how order-only prequisites are used.)

[1]: http://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html

[2]: http://www.kolpackov.net/pipermail/notes/2004-January/000001.html

Closes #6142

The .tmp files were missed before. I don't think there's a need to use
*.ext instead of just *.

Most of our documentation requires both pandoc and node.js.
This simplifies the logic around those checks and fixes an
error when building without node.js but with pandoc.

There's no reason to make them executable, and this throws a warning
with the Arch Linux package lint tool (namcap) for .a files.

Closes #5643

This also removes the need to pass noexecstack to gcc, but that wasn't
actually working anymore.

Closes #4468

Can make turnaround of testing changes to core/std/syntax much faster.