ARM: add backend support for the ABI used in WatchOS

At the LLVM level this ABI is essentially a minimal modification of AAPCS to
support 16-byte alignment for vector types and the stack.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251570 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed