[libc] Simplify enabling the GPU build for libc

Currently the GPU build requires the `LLVM_LIBC_FULL_BUILD` option to be
set. This patch changes the logic so that it is always enabled when
targeting the GPU. Also, this patch allows `LIBC_GPU_BUILD` and
`LIBC_GPU_ARCHITECTURES` to both enable a GPU build. Now, enabling the
GPU support should only require the following CMake:

```
  -DLLVM_ENABLE_RUNTIMES=libc -DLIBC_GPU_ARCHITECTURES=gfx1030
```

Reviewed By: jdoerfert, sivachandra

Differential Revision: https://reviews.llvm.org/D146979