newlib: update build system generation documentation

newlib: update build system generation documentation

Replace all of the individual autotool steps with a single autoreconf.
This simplifies the documentation greatly, and in the current system,
only takes ~10 seconds to regenerate everything.

Update the developer documentation to cover all the major components
of the current build system.  Hopefully this is a fairly complete road
map to everything.  I tried to include everything that I wish I knew
when I started hacking on this :P.