Fix Exception#detailed_message for GC compaction

	Before this commit, the test fails with RGENGC_CHECK_MODE enabled:

	    TestException#test_detailed_message_under_gc_compact_stress [test/ruby/test_exception.rb:1466]:
	    <"\e[1mfoo (\e[1;4mRuntimeError\e[m\e[1m)\e[m\n" +
	    "\e[1mbar\e[m\n" +
	    "\e[1mbaz\e[m"> expected but was
	    <"\e[1mfoo (\e[1;4mRuntimeError\e[m\e[1m)\e[m\n" +
	    "\e[1m\x00\x00\x00\x00\x00\x00\x00\e[m">.

assert_separately adds --disable=gems so we don't need to add
--disable-gems when calling assert_separately.

[Feature #19755]

Before (in /tmp/test.rb):

```ruby
Object.class_eval("p __FILE__") # => "(eval)"
```

After:

```ruby
Object.class_eval("p __FILE__") # => "(eval at /tmp/test.rb:1)"
```

This makes it much easier to track down generated code in case
the author forgot to provide a filename argument.

Raise a `SyntaxError` with the parser error message, in the case
reading from a file instead of the `-e` option or standard input. So
syntax_suggest can get the message from the caught error.

Build and store the error message with `#detailed_message` before
terminating all Ractors, then show the message later.