newlib: libc: merge build up a directory

newlib: libc: merge build up a directory

Convert all the libc/ subdir makes into the top-level Makefile.  This
allows us to build all of libc from the top Makefile without using any
recursive make calls.  This is faster and avoids the funky lib.a logic
where we unpack subdir archives to repack into a single libc.a.  The
machine override logic is maintained though by way of Makefile include
ordering, and source file accumulation in libc_a_SOURCES.

There's a few dummy.c files that are no longer necessary since we aren't
doing the lib.a accumulating, so punt them.

The winsup code has been pulling the internal newlib ssp library out,
but that doesn't exist anymore, so change that to pull the objects.