Fix build-script for macOS-Android cross-compilation SDK

[marcprux]

PR #162 adds building and testing the Android SDK on macOS. It currently just re-uses the Ubuntu SDK that is created and cached in the CI. Ideally, we would also be able to build the SDK from macOS by running build-script in the macOS CI job.

However, there are various errors when running the build script. One such error (from a CI Run) is:

FAILED: lib/swift/android/x86_64/libswiftRemoteMirror.so 
: && /Users/runner/work/swift-android-sdk/swift-android-sdk/sdk-config/swift-5.10.1-RELEASE/usr/bin/clang -Wno-unknown-warning-option -Werror=unguarded-availability-new -fno-stack-protector -fPIC -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -O3 -DNDEBUG    -shared -Wl,-soname,libswiftRemoteMirror.so -target x86_64-unknown-linux-android24 --sysroot=/Users/runner/Library/Android/sdk/ndk/26.3.11579264/toolchains/llvm/prebuilt/darwin-x86_64/sysroot -B /Users/runner/Library/Android/sdk/ndk/26.3.11579264/toolchains/llvm/prebuilt/darwin-x86_64/bin -lm -resource-dir= -fuse-ld=lld -Wl,-dead_strip -o lib/swift/android/x86_64/libswiftRemoteMirror.so   stdlib/public/LLVMSupport/CMakeFiles/swiftLLVMSupport-android-x86_64.dir/ErrorHandling.cpp.o stdlib/public/LLVMSupport/CMakeFiles/swiftLLVMSupport-android-x86_64.dir/Hashing.cpp.o stdlib/public/LLVMSupport/CMakeFiles/swiftLLVMSupport-android-x86_64.dir/MemAlloc.cpp.o stdlib/public/LLVMSupport/CMakeFiles/swiftLLVMSupport-android-x86_64.dir/SmallPtrSet.cpp.o stdlib/public/LLVMSupport/CMakeFiles/swiftLLVMSupport-android-x86_64.dir/SmallVector.cpp.o stdlib/public/LLVMSupport/CMakeFiles/swiftLLVMSupport-android-x86_64.dir/StringRef.cpp.o stdlib/public/SwiftRemoteMirror/CMakeFiles/swiftRemoteMirror-android-x86_64.dir/SwiftRemoteMirror.cpp.o -L/Users/runner/work/swift-android-sdk/swift-android-sdk/sdk-config/build/Ninja-Release/llvm-android-x86_64/./lib   -L/Users/runner/work/swift-android-sdk/swift-android-sdk/sdk-config/build/Ninja-Release/swift-android-x86_64/./lib/swift/android/x86_64   -L/Users/runner/work/swift-android-sdk/swift-android-sdk/sdk-config/swift-5.10.1-RELEASE/usr/bin/../lib/swift/android/x86_64   -L/Users/runner/work/swift-android-sdk/swift-android-sdk/sdk-config/swift-5.10.1-RELEASE/usr/bin/../lib/swift/android   -L/Users/runner/Library/Android/sdk/ndk/26.3.11579264/toolchains/llvm/prebuilt/darwin-x86_64/sysroot/usr/lib/swift -Wl,-rpath,"\$ORIGIN"  lib/swift/android/x86_64/libswiftRemoteInspection.a  -ldl  -llog  /Users/runner/Library/Android/sdk/ndk/26.3.11579264/toolchains/llvm/prebuilt/darwin-x86_64/sysroot/usr/lib/x86_64-linux-android/libc++abi.a  /Users/runner/Library/Android/sdk/ndk/26.3.11579264/toolchains/llvm/prebuilt/darwin-x86_64/sysroot/usr/lib/x86_64-linux-android/libc++_shared.so && :
ld.lld: error: unknown argument '-dead_strip'
ld.lld: error: cannot open lib/linux/libclang_rt.builtins-x86_64-android.a: No such file or directory
ld.lld: error: unable to find library -l:libunwind.a
ld.lld: error: cannot open lib/linux/libclang_rt.builtins-x86_64-android.a: No such file or directory
ld.lld: error: unable to find library -l:libunwind.a

I suspect that many of these come from cmake checking whether the host system is Darwin without accounting for the destination.