mips32, change in pracc_list for dynamic allocation

pracc_list points to an array with code in the lower half
and addr in the upper half. Change it to a struct with
an instruction field and an address field.
Requiered to make reallocation easier.
As a side effect the code is less quirky.

Change-Id: Ibf904a33a2f35a7f69284d2a2114f4b4ae79219f
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/4019


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>