x264 G5 variant

[jmcintyre]

G5 based computers see a significant performance boost in x264 if the compiler is told to build specifically for it. I've created a simple variant for the G5.

variant G5 {

configure.cflags-append "-mcpu=970 -mtune=970 -mpowerpc64 -mpowerpc-gpopt -fast -mabi=altivec -maltivec"

}

Most of the improvements come from the first four parameters. You could argue that -fast isn't really a G5 specific tag, so it shouldn't be here, but since this is trying to tune as tightly for performance, I think it is worth having. I can make a patch file if it is needed.