FreeBSD_HEAD_arm64 - Build #2305 - Fixed

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FreeBSD_HEAD_arm64 - Build #2305 - Fixed:

Build information: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_arm64/2305/
Full change log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_arm64/2305/changes
Full build log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_arm64/2305/console

Change summaries:

295250 by br:
Fix build.

295249 by kib:
Guard against runnable td2 exiting and than being reused for unrelated
process when the parent sleeps waiting for the debugger attach on
fork.

Diagnosed and reviewed by:	mjg
Sponsored by:	The FreeBSD Foundation