Fix build failure with bundled gc on Darwin 11

[ryandesign]

The patch no longer applied:

./configure --with-provided-gc

*****
***** GC support
*****
**** Patching GC library
can't find file to patch at input line 4
Perhaps you used the wrong -p or --strip option?
The text leading up to this was:
--------------------------
|diff -r a63253e53bf9 gc/libatomic_ops-1.2/src/atomic_ops/sysdeps/gcc/x86.h
|--- a/gc/libatomic_ops-1.2/src/atomic_ops/sysdeps/gcc/x86.h    Sat Oct 15 23:51:26 2011 +0200
|+++ b/gc/libatomic_ops-1.2/src/atomic_ops/sysdeps/gcc/x86.h    Sun Oct 16 00:05:12 2011 +0200
--------------------------
File to patch: 

The issues being fixed were already addressed in newer versions of gc:

The problem in os_dep.c was fixed in gc 7.2 via ivmai/bdwgc@c1cac9e179a5ea2e7cbb8b9b71a1ea46ebd3b3da.

The problem in mach_dep.c was fixed in gc 7.3 via ivmai/bdwgc@333f2e13d5d8c0c1129de13cb822db4f559b313d.

The libatomic_ops problem was fixed in 7.2d via ivmai/libatomic_ops@d923334022938240c43c167d66549b1211aed2ba.

libatomic_ops is no longer bundled as of gc 7.4.

[egallesio]

Thanks for this PR. I merge it immediately.