Implement ARM pcs attribute. Basically it's another way of calling convention selecti...

Implement ARM pcs attribute. Basically it's another way of calling convention selection (AAPCS or
AAPCS+VFP), similar to fastcall / stdcall / whatevercall seen on x86.

In particular, all library functions should always be AAPCS regardless of floating point ABI used.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@129534 91177308-0d34-0410-b5e6-96231b3b80d8