libstdc++: editorconfig: Adjust wildcard patterns

According to the editorconfig file format description, a match against
one of multiple different strings is described with those different
strings separated by commas and within curly braces.  E.g.
    [{x,y}.txt]

https://editorconfig.org/

, under "Wildcard Patterns".

The current libstdc++-v3/.editorconfig file has a few places where we
match against similar globs by using strings separated by commas but
without the curly braces.  E.g.
    [*.h,*.cc]

This doesn't take affect in neovim nor emacs (as far as I can tell), I
haven't looked into other editors.
I would expect that following the standard syntax described in the
documentation would satisfy more editors.  Hence this patch suggests
following that standard by using something like:
    [*.{h,cc}]

libstdc++-v3/ChangeLog:

	* .editorconfig: Adjust globbing style to standard syntax.

Signed-off-by: Matthew Malcomson <mmalcomson@nvidia.com>