Use the GCC builtin functions for the non-inlined signbit implementations.

2015-09-18  Wilco Dijkstra  <wdijkstr@arm.com>

        * sysdeps/ieee754/dbl-64/s_signbit.c (__signbit):
        Use __builtin_signbit.
        * sysdeps/ieee754/flt-32/s_signbitf.c (__signbitf):
        Use __builtin_signbitf.
        * sysdeps/ieee754/ldbl-128/s_signbitl.c (__signbitl):
        Use __builtin_signbitl.
        * sysdeps/ieee754/ldbl-128ibm/s_signbitl.c (___signbitl): Likewise.
        * sysdeps/ieee754/ldbl-96/s_signbitl.c (__signbitl): Likewise.