AArch64: support 'c' and 'n' inline asm modifiers.

These are tested by test/CodeGen/Generic, so we should probably know
how to deal with them. Fortunately generic code does it if asked.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209646 91177308-0d34-0410-b5e6-96231b3b80d8