target/microblaze: Move bimm to BIMM_FLAG

It makes sense to keep BIMM with D_FLAG, as they can be written
back to iflags at the same time.  BIMM_FLAG does not need to be
added to IFLAGS_TB_MASK because it does not affect the next TB,
only the exception path out of the current TB.  Renumber IMM_FLAG,
as the value 4 holds no particular significance; pack these two
flags at the bottom of the bitfield.

Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>