package/sdl2: disable altivec built-in function for PowerPC

As reported in this bug report [1], altivec support in SDL break
arbitrary C++ code.

Issue reported by test-pkg script while testing supertux package:
    error: could not convert 'true' from 'bool' to '__vector(4) __bool int'

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi/?bug=770670



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>