Remove useless SPARC signbit aliases.

Remove useless SPARC signbit aliases.

The SPARC implementations of __signbit* functions have aliases
signbit, signbitf, signbitl.  These are useless, as they aren't
exported from the shared libraries (only the __signbit* functions are
exported, to be used by the type-generic signbit macro with older
compilers).  This patch removes the useless aliases.

Tested (compilation only) with build-many-glibcs.py for
sparc64-linux-gnu and sparcv9-linux-gnu.

	* sysdeps/sparc/sparc32/fpu/s_signbit.S (signbit): Remove alias.
	(signbitf): Likewise.
	* sysdeps/sparc/sparc64/fpu/multiarch/s_signbit.S (signbit):
	Likewise.
	(signbitl): Likewise.
	* sysdeps/sparc/sparc64/fpu/multiarch/s_signbitf.S (signbitf):
	Likewise.
	* sysdeps/sparc/sparc64/fpu/s_signbit.S (signbit): Likewise.
	(signbitl): Likewise.
	* sysdeps/sparc/sparc64/fpu/s_signbitf.S (signbitf): Likewise.