newlib: libm: merge machine/ trampoline up a level

newlib: libm: merge machine/ trampoline up a level

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

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