bug.h, compiler.h: introduce compiletime_assert & BUILD_BUG_ON_MSG

bug.h, compiler.h: introduce compiletime_assert & BUILD_BUG_ON_MSG

Introduce compiletime_assert to compiler.h, which moves the details of how
to break a build and emit an error message for a specific compiler to the
headers where these details should be.  Following in the tradition of the
POSIX assert macro, compiletime_assert creates a build-time error when the
supplied condition is *false*.

Next, we add BUILD_BUG_ON_MSG to bug.h which simply wraps
compiletime_assert, inverting the logic, so that it fails when the
condition is *true*, consistent with the language "build bug on." This
macro allows you to specify the error message you want emitted when the
supplied condition is true.

Finally, we remove all other code from bug.h that mucks with these details
(BUILD_BUG & BUILD_BUG_ON), and have them all call BUILD_BUG_ON_MSG.  This
not only reduces source code bloat, but also prevents the possibility of
code being changed for one macro and not for the other (which was
previously the case for BUILD_BUG and BUILD_BUG_ON).

Since __compiletime_error_fallback is now only used in compiler.h, I'm
considering it a private macro and removing the double negation that's now
extraneous.

Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Joe Perches <joe@perches.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>