Patch ping

[Paolo Bonzini]

> http://gcc.gnu.org/ml/gcc-patches/2004-06/msg00049.html
> Re: [Patch/RFC] Enable PCH for mingw32

This is a very simple patch to add the extension .gch to the libstdc++ 
PCH files (not only the directory).  See also 
http://gcc.gnu.org/ml/gcc-patches/2004-06/msg00031.html saying

 > There is still a bug that I tripped over when testing the libstdc++
 > pch file.
 >
 > On targets that HAVE_TARGET_EXECUTABLE_SUFFIX the command
 >  gcc foo.h -ofoo
 > creates foo.{TARGET_EXEXUTABLE_SUFFIX) not foo.  The problem arise in
 > the call to convert_filename in gcc.c.

It is mostly aesthetic because PCH actually works with libstdc++-v3 on 
mingw32.

Paolo