main: use wasm-ld instead of wasm-ld-8 on macOS

This commit does a few things:
  * remove the -8 suffix on macOS, where it is not necessary
  * add smoke tests for compiling wasm files on Linux and macOS

commands_macos.go

@@ -7,5 +7,5 @@ var commands = map[string]string{
 	"ar":      "llvm-ar",
 	"clang":   "clang-8",
 	"ld.lld":  "ld.lld-8",
-	"wasm-ld": "wasm-ld-8",
+	"wasm-ld": "wasm-ld",
 }