sim: mips: Add shadow mappings for 32-bit memory address space

32-bit MIPS programs run on the 64-bit simulator model in 64-bit
sign-extended space. The mapping from 64-bit sign-extended addresses to
32-bit addresses was removed by commit
26f8bf63, breaking the 64-bit simulator
model. Add shadow mappings from 64-bit sign extended address space to
32-bit address spaces, in lieu of the AddressTranslation function.

2021-05-04  Faraz Shahbazker  <fshahbazker@wavecomp.com>

sim/mips/ChangeLog:
	* interp.c (sim_open): Add shadow mappings from 32-bit
	address space to 64-bit sign-extended address space.