fix: pre-link libgcc_eh.a explicitly so its _Unwind_Resume wins over libgcc_s stubs

GCC 15 MinGW spec adds -lgcc_s alongside -lgcc_eh even when -static-libgcc is
set. With -Wl,-Bstatic active, -lgcc_s resolves to libgcc_s.a (MSYS2 ships a
static archive of DLL import stubs) which also contains _Unwind_Resume, causing
a duplicate definition error.

Fix: explicitly link -lgcc_eh and -lgcc in Bstatic mode at the very start of
extra-ldflags, before codec libs and before GCC's spec-added libs. The static
EH implementation lands first in the symbol table; --allow-multiple-definition
then silently ignores the duplicate stubs from libgcc_s.a that arrive later.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: alfg

scripts/build-ffmpeg-windows.sh

@@ -61,7 +61,7 @@ cd "$SRC/ffmpeg"
   --pkg-config=pkg-config \
   --pkg-config-flags="--static" \
   --extra-cflags="-I$PREFIX/include" \
-  --extra-ldflags="-L$PREFIX/lib -static-libgcc -static-libstdc++ -Wl,-Bstatic -Wl,--start-group" \
+  --extra-ldflags="-L$PREFIX/lib -Wl,-Bstatic,-lgcc_eh,-lgcc -Wl,--allow-multiple-definition -static-libgcc -static-libstdc++ -Wl,--start-group" \
   --extra-ldexeflags="-Wl,--end-group -Wl,-Bdynamic" \
   --extra-libs="-lm" \
   --target-os=mingw32 \