[ARM] Define a subtarget feature and use it to decide whether long calls should

[ARM] Define a subtarget feature and use it to decide whether long calls should
be emitted.

This is needed to enable ARM long calls for LTO and enable and disable it on a
per-function basis.

Out-of-tree projects currently using EnableARMLongCalls to emit long calls
should start passing "+long-calls" to the feature string (see the changes made
to clang in r241565).

rdar://problem/21529937

Differential Revision: http://reviews.llvm.org/D9364

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241566 91177308-0d34-0410-b5e6-96231b3b80d8