[libc] Tweak the runtimes cross-build for GPU (#178548)
Summary:
We should likely use `-DLLVM_DEFAULT_TARGET_TRIPLE` as the general
source of truth, make the handling work with that since we use it for
the output directories. Fix the creation of startup files in this mode
and make sure it can detect the GPU properly.
Fixes: https://github.com/llvm/llvm-project/issues/179375
(cherry picked from commit e07a1182fd58a5b48a2c78bc3ae03872186d4ae0)
