Pull up following revision(s) (requested by riastradh in ticket #264):

sys/arch/ia64/ia64/vm_machdep.c: revision 1.18
sys/arch/powerpc/powerpc/locore_subr.S: revision 1.67
sys/arch/aarch64/aarch64/locore.S: revision 1.91
sys/arch/mips/include/asm.h: revision 1.74
sys/arch/hppa/include/cpu.h: revision 1.13
sys/arch/arm/arm/armv6_start.S: revision 1.38
sys/arch/evbmips/ingenic/cpu_startup.S: revision 1.2
sys/arch/mips/mips/locore.S: revision 1.229
sys/arch/aarch64/aarch64/cpuswitch.S: revision 1.40
sys/arch/alpha/include/asm.h: revision 1.45
sys/arch/sparc64/sparc64/locore.s: revision 1.432
sys/arch/vax/vax/subr.S: revision 1.42
sys/arch/mips/mips/locore_mips3.S: revision 1.116
sys/arch/riscv/riscv/cpu_switch.S: revision 1.3
sys/arch/ia64/ia64/machdep.c: revision 1.44
sys/arch/arm/arm32/cpuswitch.S: revision 1.106
sys/arch/sparc/sparc/locore.s: revision 1.284

aarch64: Add missing barriers in cpu_switchto.
Details in comments.

Note: This is a conservative change that inserts a barrier where
there was a comment saying none is needed, which is probably correct.
The goal of this change is to systematically add barriers to be
confident in correctness; subsequent changes may remove some bariers,
as an optimization, with an explanation of why each barrier is not
needed.

PR kern/57240

alpha: Add missing barriers in cpu_switchto.
Details in comments.

arm32: Add missing barriers in cpu_switchto.
Details in comments.

hppa: Add missing barriers in cpu_switchto.
Not sure hppa has ever had working MULTIPROCESSOR, so maybe no
pullups needed?

ia64: Add missing barriers in cpu_switchto.
(ia64 has never really worked, so no pullups needed, right?)

mips: Add missing barriers in cpu_switchto.
Details in comments.

powerpc: Add missing barriers in cpu_switchto.
Details in comments.

riscv: Add missing barriers in cpu_switchto.
Details in comments.

sparc: Add missing barriers in cpu_switchto.

sparc64: Add missing barriers in cpu_switchto.
Details in comments.

vax: Note where cpu_switchto needs barriers.

Not sure vax has ever had working MULTIPROCESSOR, though, and I'm not
even sure how to spell store-before-load barriers on VAX, so no
functional change for now.

(martin)