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

[ARM] Define subtarget feature "reserve-r9", which is used to decide
whether register r9 should be reserved.

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242737 91177308-0d34-0410-b5e6-96231b3b80d8