ffmpeg_dec: enable FFmpeg asm + repoint to sof-hifi for Xtensa HiFi float_dsp

Activate the Xtensa HiFi AVFloatDSPContext kernels (lgirdwood/FFmpeg sof-hifi,
ff_float_dsp_init_xtensa) in the ffmpeg_dec module build:

  - ffmpeg.cmake: stop passing --disable-asm to FFmpeg configure. FFmpeg treats
    every per-arch optimisation dir -- including our C-intrinsic libavutil/xtensa/
    -- as "asm"; --disable-asm forces arch=c and drops them (ARCH_XTENSA=0). There
    is no Xtensa assembly, so leaving asm enabled is safe and is what makes
    ARCH_XTENSA=1 and builds ff_float_dsp_init_xtensa.
  - west.yml: bump ffmpeg to sof-hifi @ a600a78acd (adds libavutil/xtensa HiFi
    float_dsp on top of the av_tx table cap).

Verified: pristine ptl (intel_adsp/ace30/ptl) AAC build (FLAC+AAC) reconfigures
FFmpeg with ARCH_XTENSA=1, compiles libavutil/xtensa/float_dsp_init.o, links it
into ffmpeg_dec.llext with no errors or dangerous relocations. Under the
Zephyr-SDK GCC the kernels run as the scalar C fallback (GCC lacks xt_hifi4.h);
the xtfloatx2 SIMD path activates with an xt-clang/XCC build for the ace30 core.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lrgirdwo

src/audio/ffmpeg_dec/ffmpeg.cmake

@@ -180,7 +180,12 @@ ExternalProject_Add(ffmpeg_ext
 			--prefix=${FFMPEG_INSTALL_DIR}
 			--enable-cross-compile --target-os=none --arch=xtensa
 			--cross-prefix=${_ff_cross_prefix} --cc=${CMAKE_C_COMPILER}
-			--disable-asm --disable-all --disable-everything --disable-autodetect
+			# NOTE: do NOT pass --disable-asm. FFmpeg treats every per-arch
+			# optimisation dir (incl. our C-intrinsic libavutil/xtensa/) as
+			# "asm"; --disable-asm forces arch=c and drops them. There is no
+			# Xtensa assembly, so leaving asm enabled is safe (HiFi kernels are
+			# C intrinsics) and is required to build ff_float_dsp_init_xtensa.
+			--disable-all --disable-everything --disable-autodetect
 			--disable-programs --disable-doc --disable-network
 			--disable-avformat --disable-avdevice ${_ff_avfilter_cfg}
 			--disable-swscale --disable-postproc

west.yml

@@ -66,12 +66,13 @@ manifest:
 
     # FFmpeg (libavcodec) source for the ffmpeg_dec module. Cross-built by
     # src/audio/ffmpeg_dec/ffmpeg.cmake, enabling only the Kconfig-selected
-    # decoders. SOF fork branch sof-ffmpeg-dec (release/7.1 + the av_tx table-size
-    # cap that drops the module .bss from ~17MB to ~415KB). Bump to move versions.
+    # decoders. SOF fork branch sof-hifi (release/7.1 + av_tx table-size cap that drops
+    # the module .bss from ~17MB to ~415KB, + Xtensa HiFi float_dsp). Bump to move
+    # versions.
     - name: ffmpeg
       repo-path: FFmpeg
       path: modules/audio/ffmpeg
-      revision: bc25d8d606e63c1ae8663fc0523adad908bc29e5
+      revision: a600a78acded219a98c426af0e353e6f2d689b9d
       remote: ffmpeg
 
     # libshine: small fixed-point MP3 encoder for FFMPEG_ENC_MP3 (FFmpeg has no