Add SPLIT_KERNEL_DEBUG knob

Prior to 9b6edf364eb0 WITHOUT_KERNEL_SYMBOLS split kernel debug data
into standalone debug files at build time, but did not install those
files. As of 9b6edf364eb0 it stopped splitting the debug data, leaving
it in the kernel and modules (the default kernel configs include
DEBUG=-g).

Revert 9b6edf364eb0 and introduce a new build-time SPLIT_KERNEL_DEBUG
knob, as some people rely on the pre-9b6edf364eb0 WITHOUT_KERNEL_SYMBOLS
behaviour and that was imp's original intent.

PR: 264433
Reviewed by: eugen, imp
MFC after: 3 weeks
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35437