multimedia/x265: fix build on powerpc*
A couple of fixes:
- Altivec code also uses VSX, so it should be enabled by default only
on powerpc64le. Additionaly ENABLE_ALTIVEC needs to be specified along
with CPU_POWER8.
- Altivec code is 64-bit only, so the option should be removed on
powerpc altogether. On powerpc64 it should stay non-default.
- Altivec code works only with 8 bits and causes build issues
elsewhere.