sys/modules/hwpmc/Makefile: tidy up

This module has several source files, with many conditional on the
platform architecture. Make it easier to read, and better for future
diffs against these lists.

• Convert to one SRC per line
• Simplify arm/armv7 condition
• Remove now-empty header comment
• Minor formatting

MFC after: 1 week
Sponsored by: The FreeBSD Foundation