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/apicdef.h

@@ -1,5 +1,5 @@
-#ifndef ASM_X86__APICDEF_H
-#define ASM_X86__APICDEF_H
+#ifndef _ASM_X86_APICDEF_H
+#define _ASM_X86_APICDEF_H
 
 /*
  * Constants for various Intel APICs. (local APIC, IOAPIC, etc.)
@@ -414,4 +414,4 @@ struct local_apic {
 #else
  #define BAD_APICID 0xFFFFu
 #endif
-#endif /* ASM_X86__APICDEF_H */
+#endif /* _ASM_X86_APICDEF_H */