cmd/compile: allow go:wasmimport in syscall

cmd/compile: allow go:wasmimport in syscall

The go:wasmimport compiler directive is used by the wasi port
to access host APIs, some of need to implemented in the syscall
package.

Co-authored-by: Richard Musiol <neelance@gmail.com>
Co-authored-by: Achille Roussel <achille.roussel@gmail.com>
Co-authored-by: Julien Fabre <ju.pryz@gmail.com>
Co-authored-by: Evan Phoenix <evan@phx.io>
Change-Id: I3851e154c6989094effcd25bba5864faa133564e
Reviewed-on: https://go-review.googlesource.com/c/go/+/479615
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>