PR/17383: fix gcc in-srcdir build (take 2)

[Paolo Bonzini]

> However, it *will* be somewhat confusing for developers that the location
> of the build has moved; please post an announcement to gcc@gcc.gnu.org if
> this goes in, explaining the new presence of host-i686-pc-linux-gnu, etc.

I'll do that, but consider that it only happens for in-srcdir builds. 
Out-of-srcdir does not need that (and neither do binutils or gdb).

paolo