s390: remove "64" suffix from mem64.S and swsusp_asm64.S

Rename two more files which I forgot. Also remove the "asm" from the
swsusp_asm64.S file, since the ".S" suffix already makes it obvious
that this file contains assembler code.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>