Supports compilation with GOOS=plan9 (#1603)

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/sysfs/unlink_plan9.go

@@ -0,0 +1,12 @@
+package sysfs
+
+import (
+	"syscall"
+
+	"github.com/tetratelabs/wazero/experimental/sys"
+)
+
+func unlink(name string) sys.Errno {
+	err := syscall.Remove(name)
+	return sys.UnwrapOSError(err)
+}