pixman: disable vmx for powerpc CS toolchains

The ppc CS toolchains have flaky VMX (AKA altivec) implementation
causing build failure. Disable it for these toolchains. Fixes:
http://autobuild.buildroot.net/results/3b4/3b4cda40f074578f6b35b1fb381ba37b93174e86/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>