bug#25464: 25.1.90; info-lookup support for gdb-script mode

[Eli Zaretskii]

> From: Tom Tromey <tom@tromey.com>
> Date: Mon, 16 Jan 2017 21:32:13 -0700
> 
> This patch adds support for gdb-script mode to info-lookup-symbol.
> Let me know what you think.

I like it.  But I think you should require subr-x, otherwise
string-join, which is a defsubst, won't be found, and "C-h S" will not
suggest the default symbol as expected.

Other than that, I think it's good to go.

Thanks.