newlib: libc: merge sys/ trampoline up a level

newlib: libc: merge sys/ trampoline up a level

The sys/{configure,Makefile} files exist to fan out to the specific
sys/$arch/ subdir, and to possibly generate a crt0.  We already have
all that same info in the libc/ dir itself, so by moving the recursive
configure and make calls into it, we can cut off some of this logic
entirely and save the overhead.

For arches that don't have a sys subdir, it means they can skip the
logic entirely.

The sys subdir itself is kept for the crt0 logic, for now.  We'll try
and clean that up next.