Skip libstdc++ debug build in early bootstrap stages

Skip libstdc++ debug build in early bootstrap stages

As mentioned in PR 90770, this is a patch that Debian have been carrying
for some time. The additional unoptimized copies of libstdc++ libs that
get built during each stage are never going to be used, so don't bother
building them.

For a profiled bootstrap this means we won't train the compiler on the
unoptimized library code with assertions enabled, but that doesn't seem
like a big problem, as the same code has already been compiled once for
the main libstdc++ library.

	* acinclude.m4 (GLIBCXX_ENABLE_DEBUG): Only do debug build for final
	stage of bootstrap.
	* configure: Regenerate.

From-SVN: r272509