x86/signal/64: Move 64-bit signal code to its own file

  [ bp: Fixup merge conflict caused by changes coming from the kbuild tree. ]

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lore.kernel.org/r/20220606203802.158958-9-brgerst@gmail.com


Signed-off-by: Borislav Petkov <bp@suse.de>