rcutorture: make goflag volatile

Even cmm_barrier may not be enough as a compiler barrier in the presence
of static variables, and cmm_mb would not be either!  The compiler
assumes that calling a function does not clobber static variables if it
can prove that they do not escape (which also implies the correct phase
of the moon).

Rather than sprinkling the code with compiler barriers, I am taking the
easy way out and declaring the variable volatile.  Without this patch,
rcutorture_qsbr never finishes for me (GCC 4.5.0).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>