doc, cmd/internal/obj/riscv: document the riscv64 assembler
Add documentation for the riscv64 assembler with a link to the
documentation from asm.html. Architecture specific assembler
documentation is provided for the other architectures but has
been missing for riscv64 until now.
Change-Id: I62ed7e6a2a4b52e0720d869e964b29e2a980223a
Reviewed-on: https://go-review.googlesource.com/c/go/+/652717 Reviewed-by: Joel Sing <joel@sing.id.au> Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Auto-Submit: Joel Sing <joel@sing.id.au> Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>