[mips] add some (temporary, I hope!) mips24k/mips74k hwpmc modules.

Ideally we'd have a top level hwpmc module with the shared bits, then
cpu specific glue as needed. However, on the MIPS side, there's no
probe code - {mips24k, mips74k, octeon} implement a set of methods
that hwpmc_mips.c expects.

So this populates separate modules with duplicate code.
Ew, but it does work.

This gets me off the hook - these work fine as copied into the relevant
mfsroot for mips24k/mips74k systems.

TODO:

• do it the "right" way in the future. Note that modules/hwpmc/ does build fine on MIPS, it jusn't DO anything. So it'd be nice to maybe call that "hwpmc_core" and then "hwpmc" can be the CPU/arch glue.