Fix iszero for excess precision.

Floating-point classification macros are supposed to remove any excess
range or precision from their arguments.  This patch fixes the
non-sNaN version of iszero to do so, by casting the argument to its
own type.  (This will of course work only for standard-conforming
excess precision, not for what GCC does on 32-bit x86 by default where
the back end hides excess precision from the front end; the same
applies to most of the classification macros in that case, as showed
up when we made them use GCC built-in functions.)

(iseqsig will have the reverse issue, needing to ensure that when an
underlying function is used it's for a type wide enough not to remove
any excess precision, since comparison macros must not remove excess
precision.)

Tested for x86_64 and x86.

	* math/math.h
	[__GLIBC_USE (IEC_60559_BFP_EXT) && !__SUPPORT_SNAN__] (iszero):
	Cast argument to its own type.
	* math/test-iszero-excess-precision.c: New file.
	* math/Makefile (tests): Add test-iszero-excess-precision.
	(CFLAGS-test-iszero-excess-precision.c): New variable.

math/math.h

@@ -336,7 +336,7 @@ enum
 # ifdef __SUPPORT_SNAN__
 #  define iszero(x) (fpclassify (x) == FP_ZERO)
 # else
-#  define iszero(x) ((x) == 0)
+#  define iszero(x) (((__typeof (x)) (x)) == 0)
 # endif
 #endif /* Use IEC_60559_BFP_EXT.  */