[libc] Remove remaining GPU architecture dependent instructions (#81612)

Summary:
Recent patches have added solutions to the remaining sources of
divergence. This patch simply removes the last occures of things like
`has_builtin`, `ifdef` or builtins with feature requirements. The one
exception here is `nanosleep`, but I made changes in the
`__nvvm_reflect` pass to make usage like this actually work at O0.

Depends on https://github.com/llvm/llvm-project/pull/81331