Upstream a whole host of RISC-V changes.

Upstream a whole host of RISC-V changes.

Made no attempt to separate this out into reviewable chunks, since this
is all RISC-V-specific code developed at
https://github.com/riscv/riscv-openocd

Memory sample and repeat read functionality was left out of this change
since it requires some target-independent changes that I'll upstream
some other time.

Change-Id: I92917c86d549c232cbf36ffbfefc93331c05accd
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6529
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>