mips32, add option to avoid check in last instruction

mips32, add option to avoid check in last instruction

This option is needed, for example, when exiting
debug mode in bmips targets. The last instruction
is a NOP, not a DERET. When working in async mode
this check is not done, mips32_pracc_queue_exec() pass
the parameter to mips32_pracc_exec() and never use it.

Change-Id: I4c7ed4feb1588b62e2645b955b501b6671113b36
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/4021
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>