Revert "Revert "libgc: Use GCC atomics on ARM.""

This reverts commit 490e92ffa9f4c95e20eab3706cb75108b38deb6c.
Revert "Revert "Enable GCC atomics on arm.""

This reverts commit 17f7feb9b0f6b4a391d434257dfb307db7a21391.