v: support `-o - x_test.v` too, describe `-o -` in the `v help` screen

cmd/v/help/default.txt

@@ -8,6 +8,7 @@ Examples:
    v run hello.v             Same as above but also run the produced executable immediately after compilation.
    v -cg run hello.v         Same as above, but make debugging easier (in case your program crashes).
    v -o h.c hello.v          Translate `hello.v` to `h.c`. Do not compile further.
+   v -o - hello.v            Translate `hello.v` and output the C source code to stdout. Do not compile further.
 
    v watch hello.v           Re-does the same compilation, when a source code change is detected.
                              The program is only compiled, not run.

vlib/v/builder/compile.v

@@ -107,6 +107,9 @@ fn (mut b Builder) run_compiled_executable_and_exit() {
 	if b.pref.only_check_syntax {
 		return
 	}
+	if b.pref.out_name.ends_with('/-') {
+		return
+	}
 	if b.pref.os == .ios {
 		panic('Running iOS apps is not supported yet.')
 	}