kbuild: Accept various mips sub-types in SUBARCH

uname -m on MIPS can give a number of results, such as mips64.  We
need to add another substitution to the sed call for SUBARCH in the
main Makefile.

Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>