[libc] Fix standard cross build targeting the GPU (#82724)

Summary:
The GPU target has recently been changed to support standard `libc`
build rules. This means we should be able to build for it both in
`LLVM_ENABLE_PROJECTS` mode, or targeting the runtimes directory
directly as in the LLVM `libc` documentation. Previously this failed
because the version check on the compiler was too strict and the
`--target=` options were not being set on the link jobs unless in CMake
cross compiliation mode. This patch fixes those so the following config
should work now to build the GPU target directly if using NVPTX.

```
cmake ../runtimes -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_C_COMPILER=clang \
  -DLLVM_ENABLE_RUNTIMES=libc -DLLVM_RUNTIMES_TARGET=nvptx64-nvidia-cuda \
  -DLLVM_DEFAULT_TARGET_TRIPLE=nvptx64-nvidia-cuda \
  -DLIBC_HDRGEN_EXE=/path/to/hdrgen/libc-hdrgen \
  -DLLVM_LIBC_FULL_BUILD=ON -GNinja
```