[libstdc++] Add --disable-hosted-libstdcxx, clean up conditionals

[Bill Wendling]

Also sprach Gabriel Dos Reis:
} 
} Are you also proposing we should say --enable-languages=cxx instead of
} the straight --enable-languages=c++?  Atre you also proposinig we
} rename the compiler from g++ to gxx?
} 
Please don't be ridiculous.

-- 
|| Bill Wendling            "Real Programmers have a Snoopy Calendar
|| wendling@ncsa.uiuc.edu    of '69 hanging on their wall"
|| Coding Simian                       -- Toon Moene