[SystemZ] Automatically detect zEC12 and z196 hosts

As on other hosts, the CPU identification instruction is priveleged,
so we need to look through /proc/cpuinfo.  I copied the PowerPC way of
handling "generic".

Several tests were implicitly assuming z10 and so failed on z196.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193742 91177308-0d34-0410-b5e6-96231b3b80d8