libstdc++: Add --disable-libstdcxx-hosted as an alias for hosted-libstdcxx

Most libstdc++ configure args are of the form --enable-libstdcxx-xxx but
the option to build freestanding is --disable-hosted-libstdcxx. If you
accidentally type --disable-libstdcxx-hosted then it will be ignored.

This adds --disable-libstdcxx-hosted as an alias for the existing arg,
so it works whichever way you say it. If both args are used explicitly
and their values do not agree, configure fails with an error.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ENABLE_HOSTED): Add libstdcxx-hosted
	enable arg as an alias for hosted-libstdcxx enable arg.
	* configure: Regenerate.