Add volatile to *(int *)(0) = 0; in qthreads source

"*(int *)(0) = 0;" is used in sections that should never be executed and if
somehow you did get there this causes a deliberate segfault so you can get a
good core dump. Clang complains if this are non-volatile, which breaks the
build if CHPL_DEVELOPER (warnings => errors) is set.