x86: Fix ASM_X86__ header guards

Change header guards named "ASM_X86__*" to "_ASM_X86_*" since:

a. the double underscore is ugly and pointless.
b. no leading underscore violates namespace constraints.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/include/asm/cmpxchg_64.h

@@ -1,5 +1,5 @@
-#ifndef ASM_X86__CMPXCHG_64_H
-#define ASM_X86__CMPXCHG_64_H
+#ifndef _ASM_X86_CMPXCHG_64_H
+#define _ASM_X86_CMPXCHG_64_H
 
 #include <asm/alternative.h> /* Provides LOCK_PREFIX */
 
@@ -182,4 +182,4 @@ static inline unsigned long __cmpxchg_local(volatile void *ptr,
 	cmpxchg_local((ptr), (o), (n));					\
 })
 
-#endif /* ASM_X86__CMPXCHG_64_H */
+#endif /* _ASM_X86_CMPXCHG_64_H */