Rather than define per-object rules in the Makefile, have small files
that define & include the right content. This simplifies the build
rules, and makes understanding the source a little easier (imo) as it
makes all the subdirs behave the same: you have 1 source file and it
produces 1 object. It's also about the same amount of boiler plate,
without having to define custom build rules that can fall out of sync.
We also realign the free & pvalloc definitions: common code puts these
in malloc.o & valloc.o respectively, not in free.o & pvalloc.o objects.
This will also be important as we merge the libc.a build into the top
dir since it relies on a single flat list of objects for overrides.
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