[CMake][libc] Support cross-compiling libc-hdrgen

This is useful when cross-compiling libc to another target in which
case we first need to compile libc-hdrgen for host. We rely on the
existing LLVM CMake infrastructure for that.

Differential Revision: https://reviews.llvm.org/D95205

GitOrigin-RevId: 1daaa6432e844404006aca55358e0500928d65fb
4 files changed