runtime: implement command line arguments in hosted environments

Implement command line arguments for Linux, MacOS and WASI.

testdata/env.go

@@ -5,10 +5,19 @@ import (
 )
 
 func main() {
+	// Check for environment variables (set by the test runner).
 	println("ENV1:", os.Getenv("ENV1"))
 	v, ok := os.LookupEnv("ENV2")
 	if !ok {
 		println("ENV2 not found")
 	}
 	println("ENV2:", v)
+
+	// Check for command line arguments.
+	// Argument 0 is skipped because it is the program name, which varies by
+	// test run.
+	println()
+	for _, arg := range os.Args[1:] {
+		println("arg:", arg)
+	}
 }

testdata/env.txt

@@ -1,2 +1,5 @@
 ENV1: VALUE1
 ENV2: VALUE2
+
+arg: first
+arg: second