Add support for aarch64

When compiling jack on aarch64, compilation fails because NGREGS is not
defined. Since this is only for debug output on segmentation faults, stub
the debug print out like it's been done for other platforms before.

Signed-off-by: Alexander Graf <agraf@suse.de>
Merge pull request #28 from agraf/master

Add aarch64 (64-bit arm) compile support