sparc: refactor sparc64 signbit{f} selector to C

sparc: refactor sparc64 signbit{f} selector to C

This patch refactors the sparc64 ifunc selector to a C implementation.
Also, the generic symbol is moved to its own implementation file
(s_signbit{f}-generic.S).

It also simplifies the multiarch Makefile by moving the common objects
from libm-sysdeps_routines and sysdeps_routines to a new sysdeps_call
rule and including it where required with the correct prefix.

Checked on sparc64-linux-gnu and sparcv9-linux-gnu.

	* sysdeps/sparc/sparc-ifunc.h (sparc_libm_ifunc_redirected): New
	macro.
	* sysdeps/sparc/sparc64/fpu/multiarch/Makefile (sysdep_calls): New
	rule.
	(sysdep_routines): Use sysdep_calls as base.
	(libm-sysdep_routines): Add generic rule for symbols shared with
	libc.  Add s_signbit-generic and s_signbitf-generic objects.
	* sysdeps/sparc/sparc64/fpu/multiarch/s_signbit.c: New file.
	* sysdeps/sparc/sparc64/fpu/multiarch/s_signbitf.c: Likewise.
	* sysdeps/sparc/sparc64/fpu/multiarch/s_signbit-generic.S: Likewise.
	* sysdeps/sparc/sparc64/fpu/multiarch/s_signbitf-generic.S: Likewise.
	* sysdeps/sparc/sparc64/fpu/multiarch/s_signbit.S: Remove file.
	* sysdeps/sparc/sparc64/fpu/multiarch/s_signbitf.S: Likewise.

Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>