[compiler-rt] Force ABI to libcxxabi when building cxustom libc++

Follow-up to 458ead66, which replaced the bespoke CMakeLists.txt
file for building a custom instrumented libc++ with an invocation of the
runtimes build.

In the the bespoke CMakeLists.txt, the LIBCXX_CXX_ABI setting was forced
to libcxxabi, but this was not done for the CMake invocation for the
runtimes build. This would cause CMake configuration issues on platforms
where the default LIBCXX_CXX_ABI setting is not libcxxabi, such as
FreeBSD.

Add `-DLIBCXX_CXX_ABI=libcxxabi` to that invocation, to make sure the
custom instrumented libc++ always uses the expected ABI.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D119554