[libcxxabi][cmake] Account for LIBCXXABI_TARGET_SUBDIR in test config (#138527)

This makes the logic and code structure match that of libc++, which handles this case (i.e. the target subdirectory being changed).
The `%{target}` substitution from libc++ is removed as libc++abi's config seems to be the only place it's used.

NOKEYCHECK=True
GitOrigin-RevId: 431507db52c8836f326f40f6ef6328bbf3c7bbd4
2 files changed