Stop using -O instead of -O2 for MIPS

Until clang 11 that was equivalent to -O2, but clang changed it to -O1 so
generated MIPS code will now be unnecessarily slow. It also removes a weird
special case from sys.mk.
This is similar to the D26471 change for debug kernels and should not change
anything since everything was previously building MIPS code at -O2 until the
clang 11 update.

Reviewed By: trasz
Differential Revision: https://reviews.freebsd.org/D26749