[SystemZ] Support z15 processor name

[SystemZ] Support z15 processor name

The recently announced IBM z15 processor implements the architecture
already supported as "arch13" in LLVM.  This patch adds support for
"z15" as an alternate architecture name for arch13.

The patch also uses z15 in a number of places where we used arch13
as long as the official name was not yet announced.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372435 91177308-0d34-0410-b5e6-96231b3b80d8