[rt] Use sigemptyset instead of sigfillset.

When using sigfillset, all signals are masked. Unfortunately,
this includes the signals that MPS requires, which leads to
deadlocks within MPS when it tries to stop the world.

We don't need to mask any signals here, so using sigemptyset
seems safe and is what the code did here before this code was
added.
[rt] Cleanup FreeBSD exception code.

The SEGV handler wasn't used / needed and was mostly commented out.
[rt] We no longer need chain_sigaction.

MPS is now using modern signal handling facilities so we no
longer need to work around it.
Merge pull request #482 from waywardmonkeys/signal-fixes

Signal fixes
Avoid executable stacks on Linux and FreeBSD.

Because we're generating assembly, we need for this section to
be present in the executable. We can also force it via the linker
flag.

Note that the generated .o files in the repo will need to be
regenerated with this section present at some point. (But the
linker flag keeps us safe for now.)
Merge pull request #483 from waywardmonkeys/non-executable-stacks

Avoid executable stacks on Linux and FreeBSD.
Update release notes.
Regenerate HARP support files after HARP changes

* sources/lib/run-time/harp-support/x86-freebsd/dylan-support.o,
  sources/lib/run-time/harp-support/x86-freebsd/runtime.harp,
  sources/lib/run-time/harp-support/x86-freebsd/runtime.o:
  Regenerate using harp-x86-freebsd-runtime-generator
  sources/lib/run-time/harp-support/x86-linux/dylan-support.o,
  sources/lib/run-time/harp-support/x86-linux/runtime.harp,
  sources/lib/run-time/harp-support/x86-linux/runtime.o:
  Regenerate using harp-x86-linux-runtime-generator
[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.
Bug 7440 7458: Update to LLVMDebugVersion 12

* sources/lib/llvm/llvm-debug.dylan
  ($llvm-debug-version): Update to version 12.
  (llvm-make-dbg-compile-unit): Change metadata layout to match version 12.
  (llvm-make-dbg-file-directory): New internal utility function.
  (llvm-make-dbg-file): Change metadata layout to match version 12.
  (as-file-directory): New internal utility function
  (llvm-make-dbg-function-type, llvm-make-dbg-function,
   llvm-make-dbg-lexical-block, llvm-make-dbg-basic-type,
   llvm-make-dbg-derived-type, llvm-make-dbg-composite-type): Change
   metadata layout to match version 12.
* sources/lib/llvm/tests/llvm-builder-test.dylan
  (function-test ins--dbg): Update to new llvm-make-dbg-file signature.
* sources/dfmc/llvm-back-end/llvm-emit-debug.dylan
  (llvm-source-record-dbg-file): Update to new llvm-make-dbg-file signature.