Makefile changes to support the new XCC

The 'which' part of XCC command in the main makefile was complaining if
there was no cross compiler installed during a make clean.
Fixes user debugfmt printfs

Applies the same va_args printf patch as in the kernel from d5ec386d.
Exports CFLAGS from Makelocal

Otherwise, your user/test CFLAGs won't get propagated to the sub-makes.