Declare machine-dependent functions in mach/proto/ncg

Declare machine-dependent functions in mach/proto/ncg

This breaks all machines because the declared return type void
disagrees with the implicit return type int (when I compile mach.c
with clang).  Unbreak i386, i80, i86, m68020, powerpc, vc4 by adding
the return types to mach.c.  We don't build any other machines; they
are broken since commit a46ee91 (May 19, 2013) declared void prolog()
and commit fd91851 (Nov 10, 2016) declared void mes(), with both
declarations in mach/proto/ncg/fillem.c.

Also fix mach/vc4/ncg/mach.c where type full is long, so fprintf()
must use "%ld" not "%d" to print full nlocals.