target-sparc: Revert setting cpu_dst to gen_dest_gpr

There is some read-after-write error within the OP=2 insns which
prevents setting cpu_dst to the real output register.  Until this
is found and fixed, always write to a temporary first.

Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>