Fix handling of Apple target triple when checking the ABI lists.

The check-cxx-abilist rule uses TARGET_TRIPLE to determine which
ABI list to check. However the triple on Apple contains the darwin
version which changes frequently, but libc++ doesn't need
different ABI lists for each darwin version.

This patch strips the minor version and patchlevel from TARGET_TRIPLE
before using it to determine the ABI list.

llvm-svn: 292557