[ARM] Define a subtarget feature that is used to avoid using movt/movw

[ARM] Define a subtarget feature that is used to avoid using movt/movw
pairs for 32-bit immediates.

This change is needed to avoid emitting movt/movw pairs when doing LTO
and do so on a per-function basis.

Out-of-tree projects currently using cl::opt option -arm-use-movt=0 or
false to avoid emitting movt/movw pairs should make changes to add
subtarget feature "+no-movt" (see the changes made to clang in r242368).

rdar://problem/21529937

Differential Revision: http://reviews.llvm.org/D11026

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242369 91177308-0d34-0410-b5e6-96231b3b80d8