toolchain: remove nommu option for MIPS

MIPS nommu never took off, the only MIPS processors without MMU are
microcontrollers and the only uclinux effort for them has non-upstream
patches against very very old versions.
See http://www.xiptech.com/uclinuxformips.htm



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>