cmdline: allow overriding -jsD directives (#26576)

[subhaushsingh]

The Problem

• Currently, passing -jsDFOO=1 -jsDFOO=2 triggers an error: cannot change built-in settings values with a -jsD directive.
• This happens because the first flag adds FOO to the internal settings dictionary. When the second flag appears, the compiler mistakenly thinks a protected system setting (like WASM) is being overwritten.

The Fix

• Added user_js_defines: A set in tools/cmdline.py to track which keys were introduced by the user during the current run.
• Smart Validation: The error now only triggers if a key is a built-in and hasn't already been defined by the user via -jsD in this session.

Verification

• Regression Test: Added a case to test/test_jslib.py that specifically tests duplicate/overridden -jsD flags.
• Safety Check: Confirmed that attempting to override actual system settings (e.g., -jsDWASM=0) still correctly fails as intended.

[sbc100]

Thanks!

lgtm % test comments

[sbc100]

Can you replace these two lines with:

self.do_runf('jslib/test_jslib_custom_settings.c', '1\n')

# verify that the settings can be specified more then once, and that the last one wins.
self.do_runf('jslib/test_jslib_custom_settings.c', '2\n', cflags=['-jsDCUSTOM_JS_OPTION=2'])

Then you can also delete the test/jslib/test_jslib_custom_settings.out file

[subhaushsingh]

@sbc100 It looks like the Codesize Checks are failing due to a pre-existing stale baseline on main (specifically test_codesize_cxx_lto.json showing a +206 byte drift). This seems unrelated to my -jsD logic. Could you please run the rebaseline-tests workflow on main to unblock the CI? Thanks!