[ARM] Pass subtarget feature "+strict-align".

This commit changes the driver to save subtarget feature "+strict-align" to the
IR instead of using backend option "arm-strict-align". This is needed for LTO.

Also, move the logic in ARM backend that was deciding whether strict alignment
should be forced to the front-end.

rdar://problem/21529937

http://reviews.llvm.org/D11472

llvm-svn: 243489