all: use ExitCode() call to get exit code of exited process

This is an addition that landed in Go 1.12 but we couldn't use before
because we were supporting Go back until Go 1.11. It simplifies the code
around processes a bit.

loader/loader.go

@@ -16,7 +16,6 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
-	"syscall"
 
 	"github.com/tinygo-org/tinygo/cgo"
 	"github.com/tinygo-org/tinygo/compileopts"
@@ -110,10 +109,7 @@ func Load(config *compileopts.Config, inputPkgs []string, clangHeaders string, t
 	err = cmd.Run()
 	if err != nil {
 		if exitErr, ok := err.(*exec.ExitError); ok {
-			if status, ok := exitErr.Sys().(syscall.WaitStatus); ok {
-				os.Exit(status.ExitStatus())
-			}
-			os.Exit(1)
+			os.Exit(exitErr.ExitCode())
 		}
 		return nil, fmt.Errorf("failed to run `go list`: %s", err)
 	}