Fix tc23_bogus_condwait.c testcase for s390x.

Fix tc23_bogus_condwait.c testcase for s390x.
The testcase used to cause a SIGILL because the address of the bogus
mutex  1 + (char*)&mx[0]  denotes a memory location that will eventually
appear in a compare-and-swap instruction. That insn does not allow
memory operands that are not word-aligned. Hence, the SIGILL.
With this fix both incarnations of this testcase (in helgrind and drd)
pass.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12038