In powerpc_get_pcpup(), make the inline assembly statement

volatile so that the compiler won't perform CSE. For SMP,
this may result in us accessing the wrong PCPU and as such
results in a bogus curthread value.

Note that getting curthread is not quite MP-safe in the sense
that it requires two instructions that aren't performed
atomically. The first instruction gets the address of the PCPU
structure and the second instruction dereferences that pointer
to get curthread. If a thread is switched-out in between these
instructions and switched-in on a different CPU, we still get
the wrong curthread.

sys/powerpc/include/cpufunc.h

@@ -158,7 +158,7 @@ powerpc_get_pcpup(void)
 {
 	struct pcpu	*ret;
 
-	__asm ("mfsprg %0, 0" : "=r"(ret));
+	__asm __volatile("mfsprg %0, 0" : "=r"(ret));
 
 	return(ret);
 }