[ARM] Pass subtarget feature "+long-calls" instead of passing backend option

Description

[ARM] Pass subtarget feature "+long-calls" instead of passing backend option
"-arm-long-calls".

This change allows using -mlong-calls/-mno-long-calls for LTO and enabling or
disabling long call on a per-function basis.

rdar://problem/21529937

Differential Revision: http://reviews.llvm.org/D9414

Details