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

The bug discovered while packaging supertux is now fixed by the previous
patch but the issue can still be present in other C++11 package if
-std=c++11 is used on PPC with Altivec vectorization.

This reverts commit d2903aaf.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>