add support for cross compiling on apple platforms

[ram-mohan]

partially fixes #345

Build: cmake -G Ninja -DCMAKE_OSX_ARCHITECTURES=arm64 ../ && ninja

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[ram-mohan]

@DichenZhang1 this can be merged dichen

[gregbenz]

@ram-mohan @DichenZhang1
Should this change allow compiling for MacIntel on Apple Silicon?

The following build command fails:

cmake -DBUILD_SHARED_LIBS=0 -DUHDR_WRITE_XMP=1 -DUHDR_WRITE_ISO=1 -G Ninja -DCMAKE_OSX_ARCHITECTURES=x86_64 ../

ninja

It throws a number of errors due to libjpeg not having x86 code:

[51/51] Linking CXX executable ultrahdr_app
FAILED: ultrahdr_app
: && /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -O3 -DNDEBUG -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk -Wl,-search_paths_first -Wl,-headerpad_max_install_names  CMakeFiles/ultrahdr_app.dir/examples/ultrahdr_app.cpp.o -o ultrahdr_app  libcore.a  /opt/homebrew/lib/libjpeg.a  libimage_io.a && :
ld: warning: ignoring file '/opt/homebrew/lib/libjpeg.a[117](jutils.c.o)': found architecture 'arm64', required architecture 'x86_64'