gdb: rename abbrev_cache to abbrev_table_cache

This cache holds `abbrev_table` objects, so I think it's clearer and
more consistent to name it `abbrev_table_cache`.  Rename it and
everything that goes along with it.

Change-Id: I43448c0aa538dd2c3ae5efd2f7b3e7b827409d8c
Approved-By: Tom Tromey <tom@tromey.com>
5 files changed