Create and use first-versions.h with macros for function symbol versions

[Joseph Myers]

On Fri, 21 Apr 2017, Andreas Schwab wrote:

> On Apr 21 2017, Joseph Myers <joseph@codesourcery.com> wrote:
> 
> > For example, if a platform with long double = double were to gain
> > distinct long double support now, the symbol versioning support for
> > lgamma might be more complicated.
> 
> So what's the benefit if it cannot be generalized?

It can readily be made to cover the vast bulk of the cases where special 
symbol version handling is needed for each libm function affected by a 
change of long double format, leaving only a limited number of special 
cases that may still need to do things manually.

An underlying objective is to have common macros meaning "set up all the 
aliases and symbol versioning for this binary64 function" or "set up all 
the aliases and symbol versioning for this binary128 function", so that 
changing the definitions of those macros then accomplishes a large part of 
adding TS 18661-3 *f64 / *f128 / *f32x / *f64x aliases.  To achieve that 
goal it doesn't need to cover every last function.

-- 
Joseph S. Myers
joseph@codesourcery.com