mpd: add more gcc exclusions

The codesourcery powerpc toolchains also include a gcc that's too old
for mpd to build properly. Fixes:
http://autobuild.buildroot.net/results/be0/be0aff9814ded4ba74999283530b805af01992da/



Also add toolchain comment exclusion for avr32 & sparc since it won't be
available anyway.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>