gdb: Add a SECURITY.txt document for GDB

gdb: Add a SECURITY.txt document for GDB

This commit adds a SECURITY document to GDB.  The idea behind this
document is to define what security expectations a user can reasonably
have when using GDB.  In addition the document specifies which bugs
GDB developers consider a security bug, and which are just "normal"
bugs.

Discussion for the creation of this initial version can be found here:

  https://inbox.sourceware.org/gdb-patches/877cmvui64.fsf@redhat.com/

Like any part of GDB, this is not intended as the absolute final
version, instead this is a living document, and this is just a
reasonable starting point from which we can iterate.

For now I've added this document as a text file but I am considering
merging this document into the manual at a later date, and having the
SECURITY.txt file just say "Read the manual"

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