Disable in-tree GDB by default on x86, mips, and powerpc.

GDB in ports contains all of the functionality as GDB in base
(including kgdb) for these platforms along with additional
functionality. In-tree GDB remains enabled on ARM and sparc64.
GDB in ports does not currently support kernel debugging on arm,
and ports GDB for sparc64 has not been tested (though it does
include sparc64 support).

Reviewed by: bdrewery, emaste, imp
Relnotes: yes
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D10399