[libc] Fix standalone cross compiling build for the GPU (#84042)

Summary:
This patch fixes some issues with building the GPU target manually
without the runtimes bootstrapping build. This fixes the install
directory and sets the default namespace to something more sensible if
not set. Also I got rid of the check on `-mcpu=native`. it was a neat
trick, but CMake in its INFINITE wisdom does not allow you to set link
flags on the compiler flag check. So I just went with the old tool
usage.