Make iseqsig handle excess precision.

Make iseqsig handle excess precision.

iseqsig, like other type-generic comparison macros, should behave like
a comparison operator in not removing excess range and precision from
its arguments (see C11 F.10.11).  This patch implements this by making
definitions of iseqsig appropriately conditional on
__FLT_EVAL_METHOD__ (including support for TS 18661-3 values of that
macro), with a corresponding testcase (that failed for 32-bit x86 in
the absence of the math.h changes) being added.  (Of course the
definitions may need reworking when float128 support is added, just as
with other type-generic macros.)

Tested for x86_64 and x86.

	* math/math.h [__GLIBC_USE (IEC_60559_BFP_EXT)] (iseqsig): Define
	conditional on value of [__FLT_EVAL_METHOD__].
	* math/test-iseqsig-excess-precision.c: New file.
	* math/Makefile (tests): Add test-iseqsig-excess-precision.