[builtins] Rounding mode support for addxf3/subxf3

Implement rounding mode support for addxf3/subxf3.

On architectures that implemented the support, this will access the
corresponding floating point environment register to apply the
correct rounding. For other architectures, it will keep the current
behaviour and use IEEE-754 default rounding mode (to nearest, ties
to even).

ARM32/AArch64 support implemented in this change. i386 and AMD64
will be added in a follow up change.

Differential Revision: https://reviews.llvm.org/D57143

llvm-svn: 357035