libstdc++: Simplify build targets for debug library

This rewrites the stamp-debug and build-debug targets in src/Makefile so
that each generated Makefile in the debug/$(SUBDIRS) directories is a
make target, instead of being created by a loop in the stamp-debug
recipe. The final adjustments to debug/Makefile are done as part of the
stamp-debug target instead of the build-debug target.

The advantage is that each $(SUBDIRS)/debug/Makefile now has the
corresponding $(SUBDIRS)/Makefile as a prerequisite, so they will be
regenerated if needed. Generating those can also be parallelized by
make, although those steps are very fast so that doesn't really matter.

This also removes the duplication in the stamp-debug recipe, which was
using exactly the same sed command for debug/Makefile and each
debug/$(SUBDIRS)/Makefile. That is done by adding "." to the list of
subdirectories to process. The recipes can also be simplified to use
separate shell commands per line, instead of using backslashes to join
the whole recipe into a single shell command.

Also replace 'echo `date` > stamp-xxx' with just 'date > stamp-xxx'
which is equivalent but simpler.

libstdc++-v3/ChangeLog:

	* src/Makefile.am: Simplify debug build targets.
	* src/Makefile.in: Regenerate.