target-mips: fix broken MIPS16 and microMIPS

Commit 240ce26a broke MIPS16 and microMIPS support as it didn't
care those branches and jumps don't have delay slot in
MIPS16 and microMIPS.

This patch introduces a new argument delayslot_size to the
gen_compute_branch() indicating size of delay slot {0, 2, 4}.
And the information is used to call handle_delay_slot() forcingly
when no delay slot is required.

There are some microMIPS branch and jump instructions that requires
exact size of instruction in the delay slot. For indicating
these instructions, MIPS_HFLAG_BDS_STRICT flag is introduced.

Those fictional branch opcodes defined to support MIPS16 and
microMIPS are no longer needed.

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>