x86/asm: Guard against building the 32/64-bit versions of the asm-offsets*.c file directly

Sometimes it is helpful to build a kernel compilation unit
directly, i.e.:

  make .../<filename>.i

in order to look at compiler output.

Since asm-offsets_{32,64}.c are included by asm-offsets.c and
building them directly doesn't evaluate the macros used (thus
making the preprocessor output not very useful), error out when
an attempt is made to build them. Issue a hint for the user to
build asm-offsets.c instead.
Suggested-by: Michael Matz <matz@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1418139917-12722-1-git-send-email-bp@alien8.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/asm-offsets_32.c

+#ifndef __LINUX_KBUILD_H
+# error "Please do not build this file directly, build asm-offsets.c instead"
+#endif
+
 #include <asm/ucontext.h>
 
 #include <linux/lguest.h>

arch/x86/kernel/asm-offsets_64.c

+#ifndef __LINUX_KBUILD_H
+# error "Please do not build this file directly, build asm-offsets.c instead"
+#endif
+
 #include <asm/ia32.h>
 
 #define __SYSCALL_64(nr, sym, compat) [nr] = 1,