cmd/internal/obj: instructions and registers for RISC-V

Start implementing an assembler for RISC-V - this provides register
definitions and instruction mnemonics as defined in the RISC-V
Instruction Set Manual, along with instruction encoding.

The instruction encoding is generated by the parse_opcodes script with
the "opcodes" and "opcodes-pseudo" files from (`make inst.go`):

  https://github.com/riscv/riscv-opcodes

This is based on the riscv-go port:

  https://github.com/riscv/riscv-go

Contributors to the riscv-go port are:

  Amol Bhave <ammubhave@gmail.com>
  Benjamin Barenblat <bbaren@google.com>
  Josh Bleecher Snyder <josharian@gmail.com>
  Michael Pratt <michael@pratt.im>
  Michael Yenik <myenik@google.com>
  Ronald G. Minnich <rminnich@gmail.com>
  Stefan O'Rear <sorear2@gmail.com>

This port has been updated to Go 1.13:

  https://github.com/4a6f656c/riscv-go

Updates #27532

Change-Id: I257b6de87e9864df61a2b0ce9be15968c1227b49
Reviewed-on: https://go-review.googlesource.com/c/go/+/193677
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/link.go

@@ -369,6 +369,7 @@ const (
 	ABasePPC64
 	ABaseARM64
 	ABaseMIPS
+	ABaseRISCV
 	ABaseS390X
 	ABaseWasm