[SystemZ] Add support for the .insn directive

Summary:
Add support for the .insn directive.

.insn is an s390 specific directive that allows encoding of an instruction
instead of using a mnemonic. The motivating case is some code in node.js that
requires support for the .insn directive.

Reviewers: koriakin, uweigand

Subscribers: koriakin, llvm-commits

Differential Revision: https://reviews.llvm.org/D21809

llvm-svn: 278012