fix libvpx for PowerPC and update ffmpeg accordingly

[barracuda156]

I open a new ticket referring from here: ​https://trac.macports.org/ticket/64653

So far I have been able to fix libvpx 1.5.0 and 1.6.1 to build for PowerPC.

36-47:~ svacchanda$ port -v installed libvpx
The following ports are currently installed:
  libvpx @1.6.1_0 (active) requested_variants='-universal' platform='darwin 9' archs='ppc' date='2022-02-16T21:07:00+0800'

It is quite probable later versions will build too, even though starting from 1.7.0 identical fixes fail to work (possibly introduction of powerpc64le there messes things up). I will still try to fix the latest libvpx for PowerPC and update here once/if that succeeds.

ffmpeg port file can be updated, as now it has:

    # as of 1.6.0 libvpx only supports darwin 10 or later
    if {${os.major} < 10} {
        depends_lib-delete path:lib/pkgconfig/vpx.pc:libvpx
        configure.args-replace --enable-libvpx --disable-libvpx
    }

However 1.6.1 already builds on 10.5.8 (and 10.6 PPC too).