x86/signal/compat: Move sigaction_compat_abi() to signal_64.c

Also remove the now-empty signal_compat.c.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20221219193904.190220-3-brgerst@gmail.com
Cc: Al Viro <viro@zeniv.linux.org.uk>