gdb/testsuite: restructure gdb_data_cache (lib/cache.exp)

In the next commit I want to add more information to
gdb_data_cache (see lib/cache.exp).  Specifically I want to track if
the underlying function of a caching proc calls gdb_exit or not.

Currently gdb_data_cache is an associative array, the keys of which
are the name of the caching proc.

In this commit I add a ',value' suffix to the gdb_data_cache keys.  In
the next commit I'll add additional entries with a different suffix.

There should be no noticable changes after this commit, this is just a
restructuring.