Add dependent header files

Reported by: https://ci.freebsd.org/job/FreeBSD-head-mips-build/6702/console

sys/modules: normalize .CURDIR-relative paths to SRCTOP

This simplifies make output/logic

Tested with: `cd sys/modules; make ALL_MODULES=` on amd64
MFC after: 1 month
Sponsored by: Dell EMC Isilon

[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, the

[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.

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