[libcxxabi] allow downstreams to override _LIBCPP_VERBOSE_ABORT

@asbirlea reports that Google does this for sanitizer analysis
downstream. Looks like a few #defines are added into a header which is
then injected into the build.

Reviewed By: vitalybuka, ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D150825