[NaCl] Enable ARM configuration.

nacl-regal.sh will now build all three architectures.

Also, Makefile is now quiet by default.  As is the
convension you can see full output of make by running
make -V.
Makefile: Use immediate assignement when calculating SYSTEM

This makes null builds run with just "make" go from 4.6 seconds
to 73ms, since it only calls config.guess once.
Fix GCC warnings
Enable -Werror for nacl build
Add automatic dependency generation