Use Darwin-compatible x86 assembly syntax in SMP.h (lock/cmpxchg with a slash)

ghc/includes/SMP.h

@@ -63,7 +63,7 @@ cas(StgVolatilePtr p, StgWord o, StgWord n)
 {
 #if i386_HOST_ARCH || x86_64_HOST_ARCH
     __asm__ __volatile__ (
- 	  "lock cmpxchg %3,%1"
+ 	  "lock/cmpxchg %3,%1"
           :"=a"(o), "=m" (*(volatile unsigned int *)p) 
           :"0" (o), "r" (n));
     return o;