cmd/link, runtime: support library mode on wasip1

This CL adds support of "library", i.e. c-shared, build mode on
wasip1. When -buildmode=c-shared is set, it builds a Wasm module
that is intended to be used as a library, instead of an executable.
It does not have the _start function. Instead, it has an
_initialize function, which initializes the runtime, but not call
the main function.

This is similar to the c-shared build mode on other platforms. One
difference is that unlike cgo callbacks, where Ms are created on-
demand, on Wasm we have only one M, so we just keep the M (and the
G) for callbacks.

For #65199.

Change-Id: Ieb21da96b25c1a9f3989d945cddc964c26f9085b
Reviewed-on: https://go-review.googlesource.com/c/go/+/604316


Reviewed-by: Achille Roussel <achille.roussel@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>