developers-handbook: update the section on Remote GDB

It is quite out of date, and the existing text doesn't work if followed
exactly. This should at least provide enough advice to make use of the
feature on amd64.

• Update list of required kernel options
• Mention that the GDB backend is disabled on stable and release branches
• Improve the flow of the example + remove outdated steps
• Update the example console output
• Give a tip about COPTFLAGS and <optimized out> locals
• Some minor additions/improvements to wording

Reviewed by: jhb, ygy, domagoj.stolfa_gmail.com
Approved by: ygy (docs)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31494