MFC r319873:
Move struct syscall_args syscall arguments parameters container into
struct thread.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add initial DTrace support for RISC-V.

Sponsored by: DARPA, AFRL
Sponsored by: HEIF5

Add support for ddb(4).

Sponsored by: DARPA, AFRL
Sponsored by: HEIF5

Add basic trap handlers for illegal instruction and breakpoint
exceptions.

As <machine/vmparam.h> is included from <vm/vm_param.h>, there is no
need to include it explicitly when <vm/vm_param.h> is already included.

Suggested by: alc
Reviewed by: alc
Differential Revision: https://reviews.freebsd.org/D5379

Welcome the RISC-V 64-bit kernel.

This is the final step required allowing to compile and to run RISC-V
kernel and userland from HEAD.

RISC-V is a completely open ISA that is freely available to academia
and industry.

Thanks to all the people involved! Special thanks to Andrew Turner,
David Chisnall, Ed Maste, Konstantin Belousov, John Baldwin and
Arun Thomas for their help.
Thanks to Robert Watson for organizing this project.

This project sponsored by UK Higher Education Innovation Fund (HEIF5) and
DARPA CTSRD project at the University of Cambridge Computer Laboratory.

FreeBSD/RISC-V project home: https://wiki.freebsd.org/riscv

Reviewed by: andrew, emaste, kib
Relnotes: Yes
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5
Differential Revision: https://reviews.freebsd.org/D4982