Revert r330897:

This was intended to be a non-functional change. It wasn't. The commit
message was thus wrong. In addition it broke arm, and merged crypto
related code.

Revert with prejudice.

This revert skips files touched in r316370 since that commit was since
MFCed. This revert also skips files that require $FreeBSD$ property
changes.

Thank you to those who helped me get out of this mess including but not
limited to gonzo, kevans, rgrimes.

Requested by: gjb (re)

MFC r314568 (by emaste):

kern_sig.c: ANSIfy and remove archaic register keyword

Sponsored by: The FreeBSD Foundation

MFC r318389 (by emaste):

Remove register keyword from sys/ and ANSIfy prototypes

A long long time ago the register keyword told the compiler to store
the corresponding variable in a CPU register, but it is not relevant
for any compiler used in the FreeBSD world today.

ANSIfy related prototypes while here.

Reviewed by: cem, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10193

Partial merge of the SPDX changes

These changes are incomplete but are making it difficult
to determine what other changes can/should be merged.

No objections from: pfg

MFC r315851: move thread switch tracing from mi_switch to sched_switch

MFC r315075: trace thread running state when a thread is run for the first time

MFC r312426: fix a thread preemption regression in schedulers introduced
in r270423

MFC 308565: Allow scheduling during early boot.

- Send IPI wakeups once SMP is started even if cold is true.
- Permit preemptions when cold is true.

These changes are needed for EARLY_AP_STARTUP.

MFC 308564: Don't place threads on the run queue after waking up other CPUs.

The other CPU might resume and see a still-empty runq and go back to
sleep before sched_add() adds the thread to the runq. This results
in a lost wakeup and a potential hang if the system is otherwise
completely idle.

The race originated due to a micro-optimization (my fault) in 4BSD in
that it avoided putting a thread on the run queue if the scheduler was
going to preempt to the new thread. To avoid complexity while fixing
this race, just drop this optimization. 4BSD now always sets the
"owepreempt" flag when a preemption is warranted and defers the actual
preemption to the thread_unlock of the caller the same as ULE.

MFC 303503: Don't treat NOCPU as a valid CPU to CPU_ISSET.

If a thread is created bound to a cpuset it might already be bound before
its very first timeslice, and td_lastcpu will be NOCPU in that case.

Approved by: re (gjb)