bullet: disable on microblaze

gcc doesn't finish compiling btSoftBody.cpp when optimization is
enabled.

Fixes:
http://autobuild.buildroot.net/results/6e4a9ede477b031f47f7e271c38f67f432a3573c
http://autobuild.buildroot.net/results/7dc4ab759dd3b1e3e03fe52c78e31327340bf8c9
http://autobuild.buildroot.net/results/637bae8b2be6c4c64f4f7c661cc8d4606b553538



[Thomas: add reference to gcc bug.]

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>