[libc] Add support for standalone cross compilation of libc.

One should be able to do a cross build of the libc now. For example, using
clang on a x86_64 linux host, one can build for an aarch64 linux target by
specifying -DLIBC_TARGET_TRIPLE=aarch64-linux-gnu.

Follow up changes will add a baremetal config and also appropriate
documentation about cross compiling the libc for CPU targets.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D140351