[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.

llvm-svn: 181207