Base e3709cfe6a2a introduced a SPLIT_KERNEL_DEBUG knob to control whether kernel debug is placed in standalone debug files at build time, restoring the KERNEL_SYMBOLS knob to its historical behaviour of controlling only whether or not the .debug files are installed.
Follow suit in ports and set SPLIT_KERNEL_DEBUG off. We still set KERNEL_SYMBOLS off for backwards compatibility. This could be removed once all supported base system versions have SPLIT_KERNEL_DEBUG.
Sponsored by: The FreeBSD Foundation