[SystemZ] Add a couple of missing tests

A few fp128 tests were omitted from test/CodeGen/SystemZ/fp-round-01.ll
since in early days, LLVM couldn't handle implicitly generated library
calls to functions with long double arguments on SystemZ.

This deficiency was actually long since fixed, but those tests are
still missing.  This patch adds the missing tests.  NFC.

llvm-svn: 346541