PATCH to add .gitignore

[Jason Merrill]

Apparently a .gitignore file was added to the gdb repository a while 
back, and I think it makes sense for it to be in the gcc repository as 
well.  I made a few edits to avoid ignoring things that are actually 
part of the gcc repository; this isn't strictly necessary, as the ignore 
file only applies to untracked files, but it seems appropriate all the same.

Applying to trunk.
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