contrib: Explicitly pass arch to FFmpeg on macOS x86_64.

Fixes FFmpeg configure error on macOS:

```
./configure: line 588: eval: --: invalid option
eval: usage: eval [arg ...]
make[1]: *** [contrib/ffmpeg/.stamp.ffmpeg.configure] Error 2
```

...caused by attempting to `eval --arch=x86_64=yes`.

contrib/ffmpeg/module.defs

@@ -73,6 +73,9 @@ FFMPEG.CONFIGURE.extra += \
 
 ifeq (darwin,$(BUILD.system))
     FFMPEG.CONFIGURE.extra += --disable-audiotoolbox --disable-coreimage --disable-videotoolbox
+    ifeq (x86_64,$(BUILD.arch))
+        FFMPEG.CONFIGURE.extra += --arch=x86_64
+    endif
 endif
 
 ## check against tuple: B-SYSTEM where B is { 0 | 1 } for cross-compiling flag