Stop using -O instead of -O2 for MIPS

[arichardson]

Summary

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 clang
was updated to 11.

[trasz]

share/mk/sys.mk

166

And this one?

[jrtc27]

share/mk/sys.mk

166

POSIX's c89 only gives you -O, -O0 and -O1, with additional levels merely optional.