Fix invalid target triple for MAC_UNIVERSAL builds

[wcandillon]

My apologies for this AI-generated PR. I am the maintainer of https://github.com/wcandillon/react-native-webgpu and we've always relied on ios-cmake for our CMake builds. It has been tremendously helpful. This AI-generated change helped us fix our macOS build. Below is the AI-generated description of the change:

  ## Problem
  When building for `MAC_UNIVERSAL` platform, CMake fails with the error:
  clang: error: version 'arm64' in target triple 'arm64-apple-macosx11.0-arm64' is invalid

  This occurs because the toolchain generates an invalid target triple `x86_64-arm64-apple-macosx11.0` by combining multiple architectures with dashes, which clang cannot parse.

  ## Root Cause
  The `MAC_UNIVERSAL` configuration was setting `APPLE_TARGET_TRIPLE_INT` to `${ARCHS_SPLIT}-apple-macosx${DEPLOYMENT_TARGET}` where `ARCHS_SPLIT` becomes `x86_64-arm64`. This creates an invalid target triple format that clang rejects.

  ## Solution
  Remove the target triple setting for `MAC_UNIVERSAL` builds and let CMake handle the universal build using the standard `-arch x86_64 -arch arm64` approach. This follows the same pattern used by other COMBINED builds in the toolchain.

  Universal builds should rely on the `-arch` flags rather than target triples, as:
  - Target triples are meant for single architectures
  - Universal builds are handled at the linker level by combining object files from different architectures
  - CMake's built-in universal build support works correctly without explicit target triples