[AArch64] Define subtarget feature strict-align.

[AArch64] Define subtarget feature strict-align.

This commit defines subtarget feature strict-align and uses it instead of
cl::opt -aarch64-strict-align to decide whether strict alignment should be
forced.

rdar://problem/21529937

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243516 91177308-0d34-0410-b5e6-96231b3b80d8