debian: build with emacs24 by default, disable gdb on arm64

From wookey@debian.org

     id:20140808012130.GT7605@stoneboat.aleph1.co.uk

Fixes for the port in progress of debian to arm64.
debian: re-enable atomicity tests on armhf

Commit a33ec9c seems to have fixed the problem on the armhf
porterbox (harris.debian.org).