kbuild: mips: fix sed regexp to generate asm-offset.h

Changes to Makefile.kbuild ("kbuild: add -fverbose-asm to i386
Makefile") breaks asm-offset.h file on MIPS.  Other archs possibly
suffer this change too but I'm not sure.

Here is a fix just for MIPS.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>