cmd/internal/obj/riscv,cmd/link: add support for internal cgo linking on riscv64

Make it possible to internally link cgo on riscv64, which also adds
support for SDYNIMPORT calls without external linking being required.

This reduces the time of an ./all.bash run on a Sifive Hifive Unleashed by
approximately 20% (~140 minutes down to ~110 minutes).

Change-Id: I43f1348de31672718ae8676cc82f6fdc1dfee054
Reviewed-on: https://go-review.googlesource.com/c/go/+/431104


TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Than McIntosh <thanm@google.com>