gdb: provide const-correct versions of addrmap::find and addrmap::foreach

Users of addrmap::find and addrmap::foreach that have a const addrmap
should ideally receive const pointers to objects, to indicate they
should not be modified.  However, users that have a non-const addrmap
should still receive a non-const pointer.

To achieve this, without adding more virtual methods, make the existing
find and foreach virtual methods private and prefix them with "do_".
Add small non-const and const wrappers for find and foreach.

Obviously, the const can be cast away, but if using static_cast
instead of C-style casts, then the compiler won't let you cast
the const away.  I changed all the callers of addrmap::find and
addrmap::foreach I could find to make them use static_cast.

Change-Id: Ia8e69d022564f80d961413658fe6068edc71a094