CL 603055 added basic support of wasmexport. This CL follows it
and adds stack unwinding handling. If the wasmexport Go function
returns normally, we directly return to the host. If the Go
function unwinds the stack (e.g. goroutine switch, stack growth),
we need to run a PC loop to call functions on the new stack,
similar to wasm_pc_f_loop. One difference is that when the
wasmexport function returns normally, we need to exit the loop and
return to the host.

Now a wasmimport function can call back into the Go via wasmexport.
During the callback the stack could have moved. The wasmimport
code needs to read a new SP after the host function returns,
instead of assuming the SP doesn't change.

For #65199.

Change-Id: I62c1cde1c46f7eb72625892dea41e8137b361891
Reviewed-on: https://go-review.googlesource.com/c/go/+/603836


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