[roll] Roll fuchsia [kernel][riscv64] Fill in and stub out arch-specific hooks

This fills in most of the missing files, types, and functions for
riscv64.  Most are just stubbed out, but some have implementations
of the first best guess at what they might partially look like.

It introduces the `kernel_no_userabi` GN build argument, which
when left to default will be false in the riscv64 kernel toolchain
for now. This omits all the syscall, vDSO, and userboot code so
there is no user-space support at all. Users are a myth.

This is enough to get all the kernel code compiling, with lots of
undefined symbols at link time.  Since it fails at link time, the
riscv64 kernel build is not attempted even with the GN build
argument `all_cpu_phys_boot_tests=true`.  An easy way to just
attempt the kernel build is to add this like to args.gn:
universe_package_labels += ["//zircon/kernel(//zircon/kernel:kernel_riscv64)"]

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/806290
Original-Revision: f8c925da99af22fa9da80baa25c508bdaede8895
GitOrigin-RevId: f0d4e99aff4da27f6b80bd731c7b3c749241fd62
Change-Id: Id20192bc48a81486b9775c6983cb078c14241d92