float.h: Do not define INFINITY for C2x when infinities not supported

float.h: Do not define INFINITY for C2x when infinities not supported

C2x has changed the rules for defining INFINITY in <float.h> so it is
no longer defined when float does not support infinities, instead of
being defined to an expression that overflows at translation time.
Thus, make the definition conditional on __FLT_HAS_INFINITY__ (this is
already inside a C2x-conditional part of <float.h>, because previous C
standard versions only had this macro in <math.h>).

Bootstrapped with no regressions for x86_64-pc-linux-gnu.  Also did a
spot test of the case of no infinities supported by building cc1 for
vax-netbsdelf and testing compiling the new c2x-float-11.c test with
it.

gcc/
	* ginclude/float.h (INFINITY): Define only if
	[__FLT_HAS_INFINITY__].

gcc/testsuite/
	* gcc.dg/c2x-float-2.c: Require inff effective-target.
	* gcc.dg/c2x-float-11.c: New test.