doc: update FreeBSD build guide for 12.0

Summary:
As of FreeBSD 12.0, an ancient GDB is no longer installed by default.
Instead, a modern version is available from devel/gdb, which is
currently version 8.2.x. A recent LLDB is also available.

Backport of core PR15416
Depends on D2649

Test Plan: Read the doc and build bitcoind on FreeBSD

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2650