cmd/go: more cross-package references from internal/syscall/unix

On some platforms, assembly in internal/syscall/unix references
unexported runtime symbols. Catch these references so the compiler can
generate the necessary ABI wrappers.

Fixes #28769.
Updates #27539.

Change-Id: I118eebfb8b3d907b4c3562198e6afb49854f5827
Reviewed-on: https://go-review.googlesource.com/c/149817
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Clément Chigot <clement.chigot@atos.net>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/go/internal/work/gc.go

@@ -295,9 +295,9 @@ func (gcToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, erro
 	// Gather known cross-package references from assembly code.
 	var otherPkgs []string
 	if p.ImportPath == "runtime" {
-		// Assembly in syscall and runtime/cgo references
+		// Assembly in the following packages references
 		// symbols in runtime.
-		otherPkgs = []string{"syscall", "runtime/cgo"}
+		otherPkgs = []string{"syscall", "internal/syscall/unix", "runtime/cgo"}
 	} else if p.ImportPath == "runtime/internal/atomic" {
 		// sync/atomic is an assembly wrapper around
 		// runtime/internal/atomic.