Revert "Directly generate a Mach-O dynamic library using gen_snapshot (#171626)

[vashworth]

This reverts commit 0c0148b.

Fix for #174393.

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[gemini-code-assist]

Code Review

This pull request reverts a previous change that aimed to generate Mach-O dynamic libraries directly from gen_snapshot. The reverted code restores the two-step process where gen_snapshot first creates an assembly file, which is then compiled and linked into a framework using Xcode's command-line tools (cc and clang). The changes are applied consistently across the build logic and corresponding tests. My review includes a suggestion to improve the maintainability of a new test constant.

[gemini-code-assist]

For better reusability, consider separating the output file paths from the kDefaultClang constant. This would make the constant more flexible for tests that might use different output directories. You would then need to append the output path arguments at the call sites, for example:

const FakeCommand(command: <String>[
  'xcrun', 'clang', '-arch', 'arm64',
  ...kDefaultClang,
  '-o',
  'build/foo/App.framework/App',
  'build/foo/snapshot_assembly.o',
]),

const kDefaultClang = <String>[
  '-miphoneos-version-min=13.0',
  '-isysroot',
  'path/to/sdk',
  '-dynamiclib',
  '-Xlinker',
  '-rpath',
  '-Xlinker',
  '@executable_path/Frameworks',
  '-Xlinker',
  '-rpath',
  '-Xlinker',
  '@loader_path/Frameworks',
  '-fapplication-extension',
  '-install_name',
  '@rpath/App.framework/App',
];