s390x: Fix compile warnings in test cases

s390x: Fix compile warnings in test cases

Some GCC versions emit the following warnings for some s390x-specific test
cases:

   warning: listing the stack pointer register '15' in a clobber list is
   deprecated

   warning: this 'else' clause does not
   guard... [-Wmisleading-indentation] ...this statement, but...

Fix these.

Most of inline assemblies declaring r15 as clobbered do not actually
change its value.  Only in stmg_wrap() it becomes necessary to save and
restore r15.