Fix GCC 7 build of k_standard.c.

This patch adds a default case to k_standard.c that calls
__builtin_unreachable, to avoid an uninitialized variable error from
GCC 7 (reported in
<https://sourceware.org/ml/libc-alpha/2017-08/msg01012.html>).

Tested for x86_64 (with GCC 7).

	* sysdeps/ieee754/k_standard.c (__kernel_standard): Add default
	case calling __builtin_unreachable.