libstdc++ AIX results

[David Edelsohn]

	27_io/basic_istream/extractors_arithmetic/char/12.cc is failing in
the "long double" test.  Currently "double" and "long double" are the same
on AIX.  std::numeric_limits<T>::max_exponent10 returns "308" for both.
Is v3 explicitly calling a "long double" C library function that is
failing or not working in the "double" compatibility mode?  I am not sure
what could cause "long double" to fail while the equivalent "double"
succeeds. 

Thanks, David