[AArch64] Pass subtarget feature "+reserve-x18" instead of passing backend

option "-aarch64-reserve-x18".

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/D11462

llvm-svn: 243185