add CGO_CFLAGS support (#5453)

* add CGO_CFLAGS support

* the environment variable must be split

* use shlex to handle double quotes

* wrap error

goenv/goenv.go

@@ -151,6 +151,8 @@ func Get(name string) string {
 			panic("could not find cache dir: " + err.Error())
 		}
 		return filepath.Join(dir, "tinygo")
+	case "CGO_CFLAGS":
+		return os.Getenv("CGO_CFLAGS")
 	case "CGO_ENABLED":
 		// Always enable CGo. It is required by a number of targets, including
 		// macOS and the rp2040.