c: Do not use *_IS_IEC_60559 == 2

c: Do not use *_IS_IEC_60559 == 2

A late change for C2x (addressing comments from the second round of
editorial review before the CD ballot, postdating the most recent
public working draft) removed the value 2 for *_IS_IEC_60559 (a new
<float.h> macro added in C2x).  Adjust the implementation accordingly
not to use this value.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/
	* ginclude/float.h (FLT_IS_IEC_60559, DBL_IS_IEC_60559)
	(LDBL_IS_IEC_60559): Update comment.

gcc/c-family/
	* c-cppbuiltin.cc (builtin_define_float_constants): Do not use
	value 2 for *_IS_IEC_60559.

gcc/testsuite/
	* gcc.dg/c2x-float-10.c: Do not expect value 2 for *_IS_IEC_60559.