[libc] Add an option to make `libc` only build the `libc-hdrgen` tool

[libc] Add an option to make `libc` only build the `libc-hdrgen` tool

The `libc-hdergen` tool is required for cross-builds, however some cases
can cause issues when configuring this build. This patch adds an
ovveride option `LIBC_HDRGEN_ONLY` to allow us to retain the old
(incorrect) behaviour where `libc` would not build with any other
runtimes enabled.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D151155