sim: microblaze: switch to common memory functions

sim: microblaze: switch to common memory functions

Re-use the existing memory core that handles reads/writes.

This drops support for the dumpmem command, but gdb itself has
support for dumping memory regions.

The verbose command is converted to the common --verbose flag
since only two call sites use it now.

Support for the clearstats command is dropped entirely, but no
other sim really does this, and the same thing can be done by
reloading.  If it's important (clearing cycle stats) to someone,
we can add a common function for it.