Since the move away from the global areg0, we're no longer globally
reserving areg0.  Which means our use of R7 clobbers a call-saved
register.  Shift areg0 into the windowed registers.  Indeed, choose
the incoming parameter register that it comes to us by.

This requires moving the register holding the return address elsewhere.
Choose R33 for tidiness.
Acked-by: NAurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NRichard Henderson <rth@twiddle.net>