Change -march=systemz to triple and fix test

Change -march=systemz to triple and fix test

These two test cases use -march=systemz instead of a triple. In
particular, the used file format is then based on the default host
triple. This leads to different behaviour on different platforms.

The SystemZ implementation uses the integrated assembler for a
long time now. The mature-mc-support test can be fully enabled.

Differential Revision: https://reviews.llvm.org/D68129

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373098 91177308-0d34-0410-b5e6-96231b3b80d8