Exclude atomic.S source from dist rules

The zfs package supports the option --with-config=srpm which
is used to bootstrap configure to allow the 'make srpm' target
to work. This has the advantage of allowing creation of source
rpms without having all your -devel packages installed. This
source package can then be feed back in to an automated build
farm which only installs the required packages listed by the
srpm. This ensures that all proper dependencies are expressed
by the source package, because if they are not you will get
configure/build failures.

The trouble here is that --with-config=srpm prevents the
architecture check from running resulting in TARGET_ASM_DIR
being set to the default asm-generic. The 'make dist' rule
then fails because there is no asm-generic/atomic.S file
because it is generated at build time. To handle this I
have added an empty file asm-generic/atomic.S simply as a
place holder for 'make dist'.