stepmania: fix build with GCC 14 and binutils >= 2.41

[Rutpiv]

Testing the changes

• I tested the changes in this PR: briefly

Local build testing

• I built this PR locally for my native architecture (x86_64)
• I built this PR locally for these architectures using specific masterdirs:
  • i686

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Fixes the stepmania build, broken by GCC 14 and binutils >= 2.41. The failures predate the nasm update and were discovered while validating dependent packages in #57682.

Two changes to the vendored ffmpeg:

• Backport of upstream FFmpeg commit effadce6c756 (binutils >= 2.41 inline asm fix), adapted only in the file paths to match the vendored tree; the code changes are identical to upstream.
• -fpermissive passed via the configure hook for legacy code already silenced with -w upstream. The vendored ffmpeg ignores $CFLAGS from the environment, so the flag is injected through --extra-cflags.