newlib: libc: move stdlib multiplex logic from build to source files

newlib: libc: move stdlib multiplex logic from build to source files

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.

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.

Also take the opportunity to clean up the unnecessary header deps in
here.  Automake provides dependency generation for free now.