On MIPS ext8s and ext16s ops are implemented with a dedicated
instruction only on MIPS32R2, otherwise the same kind of implementation
than at TCG level (shift left followed by shift right) is used.

Change that by only implementing the ext8s and ext16s ops on MIPS32R2 so
that optimizations can be done by the optimizer. Use an inline version to
avoid having to test again for MIPS32R2 instructions. Keep the shift
implementation for the ld/st routines.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>