PowerPC64 support for counting zeros

This adds architecture-specific support for the ppc64 of the following functions below:
* zend_mm_high_bit
* zend_mm_high_bit
The added part should be inside the or options. 

#elif defined(__GNUC__) && (defined(__arm__) ||  defined(__aarch64__) || defined(__powerpc64__))

Otherwise on __powerpc64__ there will be no __GNUC__ check.