[libcxxabi][cmake] Allow building without libcxx again

This allows `-DLLVM_ENABLE_RUNTIMES=libcxxabi` to work.

This functionality was removed in D125561 (among other such removals) as
dead code, because it was only available as part of the standalone build
before.

The functionality as added back as it was, except `target_include_directories` is used instead of the cruder `target_compile_options` (with an MSVC vs not MSVC conditional split). That is just wholly better.

Reviewed By: phosek, #libc_abi

Differential Revision: https://reviews.llvm.org/D132298