[crt] Use <gc/gc_pthread_redirects.h>.

Previously, this was calling GC_pthread_* routines. This way we don't
have to ifdef as much for MPS vs. Boehm as we merge run-time libraries.
[crt] Use %zd instead of %ld for size_t.
[rt] Remove C_TESTING code.
[rt] Remove dependency on MPS from unix-support.
[rt] This asm works for Darwin as well.
[rt] Make break.c stand apart from heap-display.c
[rt] Build heap debugging utils for MPS only.
[rt] Let collector.c build with Boehm.
[rt] Fix warnings when building with Boehm (and -W).
[rt] Move _GNU_SOURCE definition to Makefile.
[rt] Prepare to move exception code.

We can't move this to a separate file entirely at the moment due
to a couple of MPS usages.