[libc] Use a prebuilt libc-hdrgen binary if available.

This feature will primarily by used by the runtimes build. In the
runtimes build, we build libc-hdrgen once for the host and use it
to build the libc for other targets. So, the host libc-hdrgen is
like a prebuilt passed to the target builds.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D141426