[SystemZ] Add to --enable-targets=all

This patch finally enables the SystemZ target in the default build
(with --enable-targets=all).

Patch by Richard Sandiford.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181209 91177308-0d34-0410-b5e6-96231b3b80d8