FreeBSD_HEAD_arm64 - Build #1145 - Fixed

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FreeBSD_HEAD_arm64 - Build #1145 - Fixed:

Build information: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_arm64/1145/
Full change log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_arm64/1145/changes
Full build log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_arm64/1145/console

Change summaries:

287836 by emaste:
arm64: add kbd.c to the build for ukbd to fix the build

Pointy hat to:	emaste

287835 by mjg:
sysctl: switch sysctllock to a sleepable rmlock, take 2

This restores r285125. Previous attempt was reverted due to a bug in rmlocks,
which is fixed since r287833.

287833 by jhb:
Threads holding a read lock of a sleepable rm lock are not permitted
to sleep.  The rmlock implementation enforces this by disabling
sleeping when a read lock is acquired. To simplify the implementation,
sleeping is disabled for most of the duration of rm_rlock.  However,
it doesn't need to be disabled until the lock is acquired.  If a
sleepable rm lock is contested, then rm_rlock may need to acquire the
backing sx lock.  This tripped the overly-broad assertion.  Fix by
relaxing the assertion around the call to sx_xlock().

Reported by:	mjg
Reviewed by:	kib, mjg
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D3324