[libc] Disable atomic optimizations for `libc` AMDGPU builds

Recently the AMDGPU backend automatically enables a pass to optimize
atomics. This results in the LTO build taking about 10x longer in all
cases. For now we disable this by default as was the case before the
patch in D152649.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D153232