libstdc++: use grep -E instead of egrep in scripts

libstdc++: use grep -E instead of egrep in scripts

egrep has been deprecated in favor of grep -E for a long time, and the
next grep release (3.8 or 4.0) will print a warning of egrep is used.
Stop using egrep so we won't see the warning.

grep's from GNU, BSD (including Mac OS X), AIX, BusyBox all support -E
and -F.  Solaris grep doesn't support -E, but extract_symvers.in already
contains a special case for Solaris and doxygen documentation generation
is already broken on non-GNU.

libstdc++-v3/ChangeLog:

	* scripts/extract_symvers.in: Use grep -E instead of egrep.
	* scripts/run_doxygen: Likewise.

(cherry picked from commit fa4e97907fc979f550c3f02cde03d9c35f99df9b)