target-microblaze: Use the new qemu_ld/st opcodes

The ability of the new opcodes to byte-swap the memory operation
simplifies the code in and around dec_load and dec_store significantly.

Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Tested-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Cc: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>