compat: Create compat_sys_p{read,write}v64

For 32-bit ABIs which have real 64-bit registers, we don't want to
break the position argument into two.  However, we still need compat
support to deal with 32-bit pointers, so we can't just use
sys_p{read,write} directly.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>