kbuild: Disable -Wunused-but-set-variable for gcc 4.6.0

kbuild: Disable -Wunused-but-set-variable for gcc 4.6.0

[ upstream commit af0e5d565d2fffcd97d1e2d89669d627cc04e8b8 ]

Disable the new -Wunused-but-set-variable that was added in gcc 4.6.0
It produces more false positives than useful warnings.

This can still be enabled using W=1

[AK: dropped W=1 support in backport]
Signed-off-by: Dave Jones <davej@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andi Kleen <ak@linux.intel.com>