sparc64-tdep.c: Don't assign using memcpy return

sparc64-tdep.c: Don't assign using memcpy return

This:

  valbuf = memcpy (buf, valbuf, len);

causes a build failure in C++, because memcpy returns the value of
"buf" as a void *.  Instead of adding a cast, we can just do the
assignment separately.

gdb/ChangeLog:

	* sparc64-tdep.c (sparc64_store_arguments): Split assignment of
	valbuf.