sparc: Implement memcpy/mempcpy ifunc selection in C
This patch refactor the SPARC64 ifunc selector to a C implementation.
The x86_64 implementation is used as default, which resulted in common
definitions (ifunc-init.h) used on both architectures. No functional
change is expected, including ifunc resolution rules.
Checked on sparc64-linux-gnu, sparcv9-linux-gnu and x86_64-linux-gnu.