ffmpeg: Fix CPU-related settings on POWER

The defaults with the `generic` CPU target are somewhere between the ppc64 and ppc64le baseline.
More specific CPU targets don't have perfect settings either.

Just try to configure together a set of baseline-compatible settings.

pkgs/development/libraries/ffmpeg/generic.nix

@@ -795,6 +795,25 @@ stdenv.mkDerivation (
       (enableFeature withExtraWarnings "extra-warnings")
       (enableFeature withStripping "stripping")
     ]
+    ++ optionals (stdenv.hostPlatform.isPower) [
+      # FFmpeg expects us to pass `--cpu=` to pick a specific feature set to compile for. If unset, it defaults to `generic`.
+      # For POWER, the default doesn't produce baseline-compliant settings. Passing a baseline-like CPU as a target doesn't
+      # produce entirely correct settings either - POWER4 leaves AltiVec enabled, but that's only guaranteed with POWER6.
+      # Just configure together everything on our own.
+
+      # Easy: Only ppc64le's baseline is recent enough to guarantee all of these.
+      (enableFeature (stdenv.hostPlatform.isPower64 && stdenv.hostPlatform.isLittleEndian) "altivec")
+      (enableFeature (stdenv.hostPlatform.isPower64 && stdenv.hostPlatform.isLittleEndian) "vsx")
+      (enableFeature (stdenv.hostPlatform.isPower64 && stdenv.hostPlatform.isLittleEndian) "power8")
+      (enableFeature (stdenv.hostPlatform.isPower64 && stdenv.hostPlatform.isLittleEndian) "ldbrx")
+
+      # Instructions that are highly specific to that series of 32-bit embedded CPUs. Never try to enable them.
+      (enableFeature false "ppc4xx")
+
+      # I *think* enabling this on 64-bit POWER is correct? Struggling to find much info on when/where this was introduced.
+      # Definitely present on the Apple G5, and likely Freescale e5500/e6500 as well.
+      (enableFeature (stdenv.hostPlatform.isPower64) "dcbzl")
+    ]
     ++ optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
       "--cross-prefix=${stdenv.cc.targetPrefix}"
       "--enable-cross-compile"