cmake + ci: guard -arch flag against empty CMAKE_OSX_ARCHITECTURES on macOS

[Akilan-Sivakumar]

Problem

Native (non-cross-compiled) macOS builds crash:

clang++: error: invalid arch name '-arch -DHIGH_BIT_DEPTH=0'
make[2]: *** [asm.S.o] Segmentation fault: 11

Root cause

Both the ARM and ARM64 asm-compile branches in source/CMakeLists.txt unconditionally add -arch ${CMAKE_OSX_ARCHITECTURES} to the assembler args whenever APPLE is set, regardless of whether CMAKE_OSX_ARCHITECTURES is actually populated.

CMAKE_OSX_ARCHITECTURES is empty by default for a native build (per the CMake docs: "If CMAKE_OSX_ARCHITECTURES is not set, the compiler's default target architecture is used. For compilers provided by Xcode, this is the host machine's architecture."). With it unset, -arch ends up with no value, and the next flag in the argument list (-DHIGH_BIT_DEPTH=0) gets consumed as the arch name - producing the malformed clang invocation above, which fails to parse and crashes the compiler driver.

This branch was extended for ARM64/CROSS_COMPILE_ARM64 in #877 (to support cross-compiling x86_64 → arm64), but neither the pre-existing ARM branch nor the new ARM64 branch guards against the empty case, so a plain native build breaks.

Changes

This PR fixes the issue at both levels - the root cause in CMake, and defensive hardening in CI:

1. source/CMakeLists.txt

Guard both occurrences with if(APPLE AND CMAKE_OSX_ARCHITECTURES) so -arch is only added when the variable is explicitly set (i.e. when cross-compiling). When unset, clang falls back to its own documented native host-arch default.

2. .github/workflows/ci.yml

Explicitly set -DCMAKE_OSX_ARCHITECTURES=arm64 across all six macOS cmake invocations (8bit, 10bit, 12bit, and the three multilib components)