x86/build: Fix build with older GCC versions

Older GCC (observed with 4.1.x) doesn't support -Wno-override-init and
also doesn't ignore unknown -Wno-* options.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Cc: Valdis.Kletnieks@vt.edu
Fixes: 5e44258d

 "x86/build: Reduce the W=1 warnings noise when compiling x86 syscall tables"
Link: http://lkml.kernel.org/r/580E3E1C02000078001191C4@prv-mh.provo.novell.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>