[dejagnu:g++] C++11: hexfloat doesn't work

[fbedard]

hexfloat seems not to work.

Failure: FAIL: 27_io/basic_ostream/inserters_arithmetic/char/hexfloat.cc execution test

Reason: terminate called after throwing an instance of 'std::invalid_argument' what(): stod

Suspect lib issue whrere stod (strtod) doesn't support hexfloat

See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64054

[abrodkin]

Still happens in 2020.09. @claziss if that's a low-hanging fruit, can we mark it for 2021.03?

[claziss]

Upstream fixed