gdb: Convert language la_read_var_value field to a method
This commit changes the language_data::la_read_var_value function
pointer member variable into a member function of language_defn.
An interesting aspect of this change is that the implementation of
language_defn::read_var_value is actually in findvar.c. This is
partly historical, the new language_defn::read_var_value is a rename
of default_read_var_value, which was already in that file, but also,
that is the file that contains the helper functions needed by the
read_var_value method, so it makes sens that the method implementation
should continue to live there (I think).
There should be no user visible changes after this commit.