Revert "libgc: Use GCC atomics on ARM."

This reverts commit 16d2bcaa379be0c38f84492a5528c5ab6dcd7e7d.

Not compatible with gcc-4-2-1 on marvell kirkwood.
Revert "Enable GCC atomics on arm."

This reverts commit 920d873ff392c90ac9ccbc05c11072827c078384.

Conflicts:
	configure.in

This does not compile on Marvell Kirkwood platform with GCC 4.2.1