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`.
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