Add a new function attribute, 'alignstack'. It will indicate (when the backends

Add a new function attribute, 'alignstack'. It will indicate (when the backends
implement support for it) that the stack should be forcibly realigned in the
prologue (and the process reversed in the epilogue).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95945 91177308-0d34-0410-b5e6-96231b3b80d8