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/shmbuf.h

@@ -1,5 +1,5 @@
-#ifndef ASM_X86__SHMBUF_H
-#define ASM_X86__SHMBUF_H
+#ifndef _ASM_X86_SHMBUF_H
+#define _ASM_X86_SHMBUF_H
 
 /*
  * The shmid64_ds structure for x86 architecture.
@@ -48,4 +48,4 @@ struct shminfo64 {
 	unsigned long	__unused4;
 };
 
-#endif /* ASM_X86__SHMBUF_H */
+#endif /* _ASM_X86_SHMBUF_H */