[ARM] Pass subtarget feature "+reserve-r9" instead of passing backend

option "-arm-reserve-r9".

This recommits r242736, which had to be reverted because the llvm-side
change that was committed in r242737 caused the number of subtarget
features to go over the limit of 64.

This change is needed since backend options do not make it to the backend
when doing LTO and are not capable of changing the behavior of code-gen
passes on a per-function basis.

rdar://problem/21529937

Differential Revision: http://reviews.llvm.org/D11319

llvm-svn: 242755