[libc] Add a target to install libc in the full build mode.

* In the full build mode, `ninja install-libc` will install the headers as
  well the static archive named libc.a.
* In the default mode, `ninja install-llvmlibc` will only install the
  static archive libllvmlibc.a.

Reviewed By: jeffbailey

Differential Revision: https://reviews.llvm.org/D132015