[libc] Don't use '-nolibc' on the GPU build

We previously changed this to use `nolibc` to allow it to link in
compiler builtins for the CPU build. However, these options are unused
on the GPU and create a lot of noise. Furthermore, we want to use
`nogpulib` to prevent the linking in of the vendor libraries.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149753