ffmpeg: pass `-miphoneos-version-min` and `-mios-simulator-version-min` through to the assembler correctly

Fixes warnings about 'assembly-object-file.o' was build for newer iOS version (14.5) than being linked (11.0) when building an iOS application.

commit 06a55323

ffmpeg: actually use gas-preprocessor on ios

Also pass the necessary flags for iphone minimum target version
selection so that object files are built for the correct version.  This
may be important if some newer code/functionality cannot be used on
older devices due to hardware or software limitations.

commit 16f4acf6

config/ios: put iphone sdk version minimum into CPPFLAGS

This may be important for anything produced by the C Preprocesser