x86: Add an mfence macro

Provide access to this x86 instruction from C code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/include/asm/cpu.h

@@ -151,6 +151,11 @@ static inline int flag_is_changeable_p(uint32_t flag)
 	return ((f1^f2) & flag) != 0;
 }
 
+static inline void mfence(void)
+{
+	__asm__ __volatile__("mfence" : : : "memory");
+}
+
 /**
  * cpu_enable_paging_pae() - Enable PAE-paging
  *