[PATCH, newlib] Allow locking routine to be retargeted

[Freddie Chopin]

Great!

I've already managed to compile the toolchain for ARM, now I'm trying
to implement all these "retargeting" functions and required lock
objects. We'll see how it works (;

BTW - maybe it would be better to leave the functions undefined and
provide the stubs only in libnosys.a? This would be consistent with
other newlib stubs (like _sbrk_r(), _read_r() and so on)?

Regards,
FCh