Disable rarely used architecture variants in make universe by default.

[jhb]

Summary

If EXTRA_TARGETS is defined, build all supported architecture
variants. By default, build architecture variants needed to provide
code coverage or that are commonly used.

Use this to disable building of all the hard-float and little-endian
MIPS architect variants by default.

Test Plan

• make kernel-toolchains without EXTRA_TARGETS defined, need to still do a build with it defined

[rpokala]

FWIW, LGTM.

[jhb]

• Drop mipsn32 as well.