Convert init_ada_exception_catchpoint to a ctor

Convert init_ada_exception_catchpoint to a ctor

Currently, init_ada_exception_catchpoint is defined in breakpoint.c, I
presume so it can call the static describe_other_breakpoints function.
I think this is a dependency inversion.
init_ada_exception_catchpoint, being code specific to Ada catchpoints,
should be in ada-lang.c, and describe_other_breakpoints, a core
function, should be exported.

And then, we can convert init_ada_exception_catchpoint to an
ada_catchpoint ctor.

Change-Id: I07695572dabc5a75d3d3740fd9b95db1529406a1