Bump __FreeBSD_version to 1600020 after the amd64 FRED KBI change

[chris.longros_gmail.com]

Summary

6e93f5e4d693 ("amd64: FRED support") changed the kernel KBI without bumping __FreeBSD_version. On amd64 CURRENT, kldload amdgpu panics with the drm-latest-kmod package.

Test Plan

Testing performed:
amd64 16.0-CURRENT GENERIC. Pre-FRED drm-latest-kmod panics on kldload; rebuilt against post-FRED /usr/src it initializes amdgpu.

[kib]

What are the hypothetical KBI incompatibilities caused by the FRED commit?

[chris.longros_gmail.com]

In D58593#1344656, @kib wrote:

What are the hypothetical KBI incompatibilities caused by the FRED commit?

struct pcb gained six fields and grew 16 bytes so td_lkpi_task moved offset. Built modules before and after the commit read different offsets.

[kib]

Ok. I think we also need to check the lkpi pointer in the kern_thread.c asserts.

[chris.longros_gmail.com]

I think this revision is not anymore relevant as the video(4) driver bumped the version to 1600020.

[emaste]

Yeah, you can abandon it for it to close. I'll add it to the porter's handbook.