Provide the initial framework for the riscv64 assembler. For now this
only supports raw WORD instructions, but at least allows for basic
testing. Additional functionality will be added in separate changes.

Based on the riscv-go port.

Updates #27532

Change-Id: I181ffb2d37a34764a3e91eded177d13a89c69f9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/194117Reviewed-by: Cherry Zhang <cherryyz@google.com>