main: add -debug flag that controls debuginfo stripping

Debug information is useful in many case, but in the case of WebAssembly
it increases the binary size by a large amount. It also inhibits
compression of relocations. Therefore I've made debug information
optional and disabled by default.

(Debug information is still generated, but stripped from the binary at
the link step).

main_test.go

@@ -194,7 +194,7 @@ func runTest(name, target string, t *testing.T, cmdArgs, environmentVars []strin
 		PrintIR:    false,
 		DumpSSA:    false,
 		VerifyIR:   true,
-		Debug:      true,
+		EmitDWARF:  true,
 		PrintSizes: "",
 		WasmAbi:    "",
 	}