gdb: add a new build_id_equal function

Add two versions of a new function build_id_equal which can be used to
compare build-ids, then make use of these functions in GDB.  It seems
better to have a specific function for the task of comparing build-ids
rather than having a length check followed by a memcmp call.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>