sparc: refactor sparc64 __mpn_mul_1 selector to C

sparc: refactor sparc64 __mpn_mul_1 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
mul_1-generic.S).

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

	* sysdeps/sparc/sparc64/multiarch/Makefile (sysdep_routines):
	Add mul_1-generic.
	* sysdeps/sparc/sparc64/multiarch/mul_1-generic.S: New file.
	* sysdeps/sparc/sparc64/multiarch/mul_1.c: Likewise.
	* sysdeps/sparc/sparc64/multiarch/mul_1.S: Remove file.

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