include/math.h: fix warning with -Wconversion

[iomartin]

The way the __fp_type_select macro uses the _Generic expression causes gcc to throw a warning on valid code if the -Wconversion flag is used.

For example, consider the following program:

#include <math.h>
int main()
{
	double x = 1.0;
	isnan(x);
	return 0;
}

which throws a warning:

$ gcc -Wconversion a.c
a.c:5:15: warning: conversion from 'double' to 'float' may change value [-Wfloat-conversion]
    5 |         isnan(x);
      |               ^

This happens because the functions are invoked inside of the _Generic. Looking at the example of _Generic in the C11 specification, one sees that the parameters are outside of the _Generic expression (see page 79 here: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf).

Reference: https://stackoverflow.com/a/68309379

[bsdimp]

Thanks! I just puzzled through _Generic a few weeks ago and hit exactly this problem with the macros I was writing.