Fix cython_args build option handling

[cyphercodes]

Fixes #15989.

Summary

• Add configured cython_args (-Dcython_args=... / machine-file built-in option) to Cython transpile commands.
• Extend the existing Cython args test with a module that only builds when cython_args from build options is passed through.

Tests

• python run_project_tests.py --use-tmpdir --only 'cython/3 cython_args' --failfast -v — passed (1 passed, 0 failed, 0 skipped)
• python run_project_tests.py --use-tmpdir --only cython --failfast — passed (6 passed, 0 failed, 0 skipped)
• Manual setup/build/test of test cases/cython/3 cython_args with -Dcython_args='--compile-time-env BUILTIN_VALUE=1' — passed; build.ninja contains the expected --compile-time-env BUILTIN_VALUE=1 Cython args.

[dcbaker]

We're in the process of deleting get_external_args, instead use:

cy_args = self.environment.coredata.optstore.get_value_for(OptionKey('cython_args', target.subproject, target.for_machine)
assert isinstance(cy_args, list), 'for mypy
args += cy_args

I might have subproject and for_machine swapped

[cyphercodes]

Addressed the review request by reading cython_args from coredata.optstore with the target subproject/machine instead of get_external_args.

Verification:

• git diff --check
• python meson.py setup /tmp/meson-cython-args 'test cases/cython/3 cython_args' -Dcython_args='--compile-time-env BUILTIN_VALUE=1'
• ninja -C /tmp/meson-cython-args test

Note: python run_project_tests.py --only 'cython/3 cython_args' did not run the cython suite in this container because the harness skipped cython compiler detection, so I verified the touched test case directly.