gdb/mi: use std::map for MI commands in mi-cmds.c

This changes the hashmap used in mi-cmds.c from a custom structure to
std::map.  Not only is replacing a custom container with a standard
one an improvement, but using std::map will make it easier to
dynamically add commands; which is something that is planned for a
later series, where we will allow MI commands to be implemented in
Python.

There should be no user visible changes after this commit.