Reorder registers. Fix problem with ret 8.

Reorder registers.  Fix problem with ret 8.

After c5bb3be, ncg began to allocate regvars from r13 up.  I reorder
the regvars so ncg again allocates them from r31 down.  I also reorder
the other registers.

This exposed a bug in my rule for ret 8.  It was wrong if item %2 was
in r3, because I moved %1 to r3 before %2 to r4.  Fix it by adding
back an individual register class for r3 (called REG3 here, GPR3 in
c5bb3be).

Also fix my typo in mach.c that made a syntax error in assembly.