pmix/pmix3x: fix macros usage in embedded pmix3x

Use PMIX_* macros instead of OPAL_* macros
master does things differently, so this is a one-off commit

Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>

opal/mca/pmix/pmix3x/pmix/src/atomics/sys/gcc_builtin/atomic.h

@@ -60,7 +60,7 @@ static inline void pmix_atomic_mb(void)
 
 static inline void pmix_atomic_rmb(void)
 {
-#if OPAL_ASSEMBLY_ARCH == OPAL_X86_64
+#if PMIX_ASSEMBLY_ARCH == PMIX_X86_64
     /* work around a bug in older gcc versions where ACQUIRE seems to get
      * treated as a no-op instead of being equivalent to
      * __asm__ __volatile__("": : :"memory") */