[SystemZ] Add MC test cases

This adds all MC tests for the SystemZ target.

Patch by Richard Sandiford.
[SystemZ] Set up JIT/MCJIT test cases

This patch adds the necessary configuration bits and #ifdef's to set up
the JIT/MCJIT test cases for SystemZ.  Like other recent targets, we do
fully support MCJIT, but do not support the old JIT at all.  Set up the
lit config files accordingly, and disable old-JIT unit tests.

Patch by Richard Sandiford.
[SystemZ] Add configure bits

This patch wires up the SystemZ target in configure, so that it can now be
built using --enable-targets=systemz.   It is not yet included in the default
build (--enable-targets=all); this will be done by a follow-up patch.

Patch by Richard Sandiford.