Have the driver and the target code agree on what the default ABI
is for each machine. Fix up darwin tests that were testing for
aapcs on armv7-ios when the actual ABI is apcs.

Should be no user visible change without -cc1.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@223429 91177308-0d34-0410-b5e6-96231b3b80d8