Kill several GCC compiler warnings (#29642)
The bogus -Wclobbered warning "argument ‘x’ might be clobbered by
‘longjmp’ or ‘vfork’" is a gcc bug. It's been reported several times on
gcc bugzilla, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65041 for
example. It's not clear how to work around this in code so I've disabled
it as locally as possible.