Defining BUILD_BUG_ON causes redefinition warnings when adding includes of
include/linux/build_bug.h in files unrelated to x86/boot.  For example,
adding an include of build_bug.h to include/linux/bits.h shows the
following warnings:

  CC      arch/x86/boot/cpucheck.o
  In file included from ./include/linux/bits.h:22,
                   from ./arch/x86/include/asm/msr-index.h:5,
                   from arch/x86/boot/cpucheck.c:28:
  ./include/linux/build_bug.h:49: warning: "BUILD_BUG_ON" redefined
     49 | #define BUILD_BUG_ON(condition) \
        |
  In file included from arch/x86/boot/cpucheck.c:22:
  arch/x86/boot/boot.h:31: note: this is the location of the previous definition
     31 | #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
        |

The macro was added to boot.h in commit 62bd0337

 ("Top header file for
new x86 setup code"). At that time, BUILD_BUG_ON was defined in
kernel.h. Presumably BUILD_BUG_ON was redefined to avoid pulling in
kernel.h. Since then, BUILD_BUG_ON and similar macros have been split to a
separate header file.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20190811184938.1796-2-rikard.falkeborn@gmail.com