liquid-dsp: blacklist CS powerpc toolchains

They're unable to build liquid-dsp successfully resulting in an internal
compiler error. Fixes:
http://autobuild.buildroot.net/results/2ac/2acd745f4e5ec6edfd09afcd57b9ece216724c1b/



[Thomas: propagate the dependencies to the comment.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>