[RISCV] Set __GCC_HAVE_SYNC_COMPARE_AND_SWAP_x defines

The RISCV target did not set the GCC atomic compare and swap defines,
unlike other targets. This broke builds for things like glib on RISCV.

Patch by Kristof Provost (kprovost)

Differential Revision: https://reviews.llvm.org/D91784