hwpmc: build hwpmc_pmu.c and hwpmc_assign.c

[afscoelho_gmail.com]

Summary

Build the two new source files for both the loadable hwpmc module
(sys/modules/hwpmc/Makefile) and the static amd64 / i386 kernels.
The static-kernel wiring lives in the unified sys/conf/files.x86,
which both files.amd64 and files.i386 include, so a single hunk
covers both architectures. No other architecture lists the two
files: hwpmc_mod.c is linked against the no-op stubs in hwpmc_pmu.h
there, so the module continues to build everywhere.

Sponsored by: AMD
Signed-off-by: Raghavendra K T <[e-mail]>

[afscoelho_gmail.com]

This revision is superseded by D57633. Its build-wiring change is folded into the rebuilt D57633 because that wiring is the activation switch for the PMU implementation and does not form a useful standalone review boundary.