Consistency fixes for FFmpeg extension instructions

PiperOrigin-RevId: 303316976

extensions/ffmpeg/README.md

@@ -35,22 +35,22 @@ FFMPEG_EXT_PATH="$(pwd)/extensions/ffmpeg/src/main/jni"
 NDK_PATH="<path to Android NDK>"
 NDK_PATH="<path to Android NDK>"
 ```
 ```
 
 
-* Set up host platform ("darwin-x86_64" for Mac OS X):
+* Set the host platform (use "darwin-x86_64" for Mac OS X):
 
 
 ```
 ```
 HOST_PLATFORM="linux-x86_64"
 HOST_PLATFORM="linux-x86_64"
 ```
 ```
 
 
-* Configure the formats supported by adapting the following variable if needed
-  and by setting it. See the [Supported formats][] page for more details of the
-  formats.
+* Configure the decoders to include. See the [Supported formats][] page for
+  details of the available decoders, and which formats they support.
 
 
 ```
 ```
 ENABLED_DECODERS=(vorbis opus flac)
 ENABLED_DECODERS=(vorbis opus flac)
 ```
 ```
 
 
-* Fetch and build FFmpeg. For example, executing script `build_ffmpeg.sh` will
-  fetch and build FFmpeg release 4.2 for armeabi-v7a, arm64-v8a and x86:
+* Fetch and build FFmpeg. Executing `build_ffmpeg.sh` will fetch and build
+  FFmpeg 4.2 for `armeabi-v7a`, `arm64-v8a`, `x86` and `x86_64`. The script can
+  be edited if you need to build for different architectures.
 
 
 ```
 ```
 cd "${FFMPEG_EXT_PATH}" && \
 cd "${FFMPEG_EXT_PATH}" && \
@@ -63,7 +63,7 @@ cd "${FFMPEG_EXT_PATH}" && \
 
 
 ```
 ```
 cd "${FFMPEG_EXT_PATH}" && \
 cd "${FFMPEG_EXT_PATH}" && \
-${NDK_PATH}/ndk-build APP_ABI="armeabi-v7a arm64-v8a x86" -j4
+${NDK_PATH}/ndk-build APP_ABI="armeabi-v7a arm64-v8a x86 x86_64" -j4
 ```
 ```
 
 
 ## Build instructions (Windows) ##
 ## Build instructions (Windows) ##

extensions/ffmpeg/src/main/jni/build_ffmpeg.sh

@@ -85,7 +85,7 @@ make clean
     --libdir=android-libs/x86_64 \
     --libdir=android-libs/x86_64 \
     --arch=x86_64 \
     --arch=x86_64 \
     --cpu=x86_64 \
     --cpu=x86_64 \
-    --cross-prefix="${TOOLCHAIN_PREFIX}/x86_64-linux-android16-" \
+    --cross-prefix="${TOOLCHAIN_PREFIX}/x86_64-linux-android21-" \
     --nm="${TOOLCHAIN_PREFIX}/x86_64-linux-android-nm" \
     --nm="${TOOLCHAIN_PREFIX}/x86_64-linux-android-nm" \
     --strip="${TOOLCHAIN_PREFIX}/x86_64-linux-android-strip" \
     --strip="${TOOLCHAIN_PREFIX}/x86_64-linux-android-strip" \
     --disable-asm \
     --disable-asm \