selective linking of floating point support for *printf / *scanf

[Thomas Preud'homme]

> From: Joseph S. Myers [mailto:joseph@codesourcery.com]
> Sent: Tuesday, September 02, 2014 11:29 PM
> 
> Identifiers beginning with a single underscore are reserved with file
> scope.  This means an application cannot provide an external definition of
> them, because such an external definition would have file scope.  So it's
> fine for the implementation to define such identifiers and use them in the
> implementation of standard functions.

Ah yes, I mistook file scope with file scope with internal linkage. So then there
shouldn't be any problem since _printf_float and _scanf_float are only used
for external linkage, no macro refer to them.

Best regards,

Thomas