[SystemZ] Start adding z196 and zEC12 support

This first step just adds definitions for SLLK, SRLK and SRAK.
The next patch will actually make use of them during codegen.

insn-bad.s tests that some form of error is reported when using these
instructions on z10.  More work is needed to get the "instruction requires:
distinct-ops" that we'd ideally like, so I've stubbed that part out for now.
I'll come back and make it mandatory once the necessary changes are in.