[libc] Search the compiler's path for GPU utility tools (#82712)

Summary:
We need some extra tools for the GPU build. Normally we search for these
from the build itself, but in the case of a `LLVM_PROJECTS_BUILD` or
some other kind of external build, this directory will not be populated.
However, the GPU build already requires that the compiler is an
up-to-date clang, which should always have these present next to the
binary. Simply add this as a fallback search path. Generally we want it
to be the second, because it would pick up someone install and then
become stale.