add back WASM_FALLBACK constant

[PPpro]

Re: #

Changelog

• fix warnning unknown WASM_FALLBACK, reverted by Fix spine.wasm could not run in safari with iOS which version is lower than 15. #15515
  

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Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

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Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.

[minggo]

Is this constant used?

[PPpro]

no for now, but it's a feature for engine build tool, we sill need keep this constant @minggo
So that when we need it, we can use it directly