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.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292557 91177308-0d34-0410-b5e6-96231b3b80d8