Use internal __feraiseexcept in __iseqsig

Replace the use of feraiseexcept with __feraiseexcept in the helper
function __iseqsig (math/s_iseqsig_template.c).

Tested for powerpc64le, s390x, and x86_64.

	* math/s_iseqsig_template.c (__iseqsig): Use __feraiseexcept
	instead of feraiseexcept.