MCParser: Reject .balign with non-pow2 alignments.

GNU as rejects them and there are configure scripts in the wild that check if
the assembler rejects ".align 3" to determine whether the alignment is in bytes
or powers of two.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175360 91177308-0d34-0410-b5e6-96231b3b80d8