[ARM] Define subtarget feature strict-align.

This commit defines subtarget feature strict-align and uses it instead of
cl::opt -arm-strict-align to decide whether strict alignment should be
forced. Also, remove the logic that was checking the OS and architecture
as clang is now responsible for setting strict-align based on the command
line options specified and the target architecute and OS.

rdar://problem/21529937

http://reviews.llvm.org/D11470

llvm-svn: 243493