testing/newlib-stage1: New aport

Intermediate step in building a cross toolchain for bare metal targets.

This is working around cyclic dependencies between GCC and newlib. The
build order is this:

1. binutils-cross-embedded (final package)
2. gcc-cross-embedded-stage1 (intermediate step, needed for step 3)
3. newlib-stage1 (intermediate step, needed for step 4)
4. gcc-cross-embedded (final package)
5. newlib (final package)