[ARM] Define subtarget feature "reserve-r9", which is used to decide

whether register r9 should be reserved.

This recommits r242737, which broke bots because the number of subtarget
features went over the limit of 64.

This change is needed because we cannot use a backend option to set
cl::opt "arm-reserve-r9" when doing LTO.

Out-of-tree projects currently using cl::opt option "-arm-reserve-r9" to
reserve r9 should make changes to add subtarget feature "reserve-r9" to
the IR.

rdar://problem/21529937

Differential Revision: http://reviews.llvm.org/D11320

llvm-svn: 242756