[vcpkg script] vcpkg_configure_meson: Support wasm32-emscripten triplet

[past-due]

Describe the pull request

Fixes "Unsupported target architecture" error when trying to compile meson-based ports targeting the wasm32-emscripten triplet.

Tested this with the fribidi port. (Compiled, linked successfully with example project, executed.)

[past-due]

@LilyWangLL Is there anything remaining that's needed to get this merged? (On my end this works well, has been tested end-to-end with the wasm32-emscripten triplet with the fribidi port, and is working great.)

[LilyWangLL]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[past-due]

@LilyWangLL From what I can see, the failures do not appear to have anything to do with this PR. (The ports listed don't even use meson.)

[LilyWangLL]

Depends on PR #26646