Add POWER altivec support to x265

[Former user]

In x265 package, there is no SIMD optimization, neither altivec nor assembler, on POWER platform. It makes the performance of x265 encoder much lower on POWER platform. I propose to add altivec support to optimize x265 on POWER platform. Thanks.

[John Comeau]

this was added about a year after the issue was opened (not by me). whoever is in charge of closing obviated issues should do so. thanks.