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.
2c266c631a