libstdc++: Improve error reporting if PDF generation fails

If pdflatex runs out of memory the build fails with no hint what's
wrong. This adds another grep command to the makefile so that an
out-of-memory error will result in more information being shown.

As suggested in https://bugzilla.redhat.com/show_bug.cgi?id=1841056
using lualatex can be used as a workaround.

libstdc++-v3/ChangeLog:

	* doc/Makefile.am (stamp-pdf-doxygen): Also grep for
	out-of-memory error in log file.
	* doc/Makefile.in: Regenerate.