fs/compat: optional preadv64/pwrite64 compat system calls

The preadv64/pwrite64 have been implemented for the x32 ABI, in order
to allow passing 64 bit arguments from user space without splitting
them into two 32 bit parameters, like it would be necessary for usual
compat tasks.
Howevert these two system calls are only being used for the x32 ABI,
so add __ARCH_WANT_COMPAT defines for these two compat syscalls and
make these two only visible for x86.

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