buildroot

vlc: workaround microblaze gcc internal compiler error

Override gcc optimization flags with -O0. The workaround is not required
for gcc6 anymore, so we take this into account into our condition.

Fixes:

  http://autobuild.buildroot.net/results/a318f0838a6a602046e719103ac81965c0084d52

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
[Thomas: add gcc 6.x condition.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>