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

(cherry picked from commit 92fc5822a8301d0c20d292e97d1661b25f5035bc)
(cherry picked from commit e3709cfe6a2a1f7d345db2870be175c9cf2a8979)
(cherry picked from commit a7bd57de86372d834cf9312fcc4c39070653e009)