libstdc++: Fix run_doxygen for Doxygen 1.10 man page format

Doxygen switched from \fC to \fR in its man page output:
https://github.com/doxygen/doxygen/pull/10497

This breaks our script that expects \fC so change the regaulr expression
to work with either style.

libstdc++-v3/ChangeLog:

	* scripts/run_doxygen: Adjust sed pattern to match '\fR' for
	new man output that Doxygen 1.10 generates.