Update autoconf to warn when building under `wasm32-unknown-emscripten`

[brettcannon]

Bug report

Bug description:

Support for wasm32-unknown-emscripten was dropped in python/peps#3612 . As part of PEP 11, a configure error needs to be triggered when building for the platform.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• GH-113632: update configure.ac for WebAssembly support tiers #115192

[brettcannon]

This is blocked on the SC answering python/steering-council#226 .

[brettcannon]

Another option is to have a warning anytime you build on a platform that isn't supported.

[erlend-aasland]

Another option is to have a warning anytime you build on a platform that isn't supported.

We already have the following code in place:

https://github.com/python/cpython/blob/5ce193e65a7e6f239337a8c5305895cf8a4d2726/configure.ac#L961C13-L995

Removing this line...:

cpython/configure.ac

Line 982 in 5ce193e

[wasm32-unknown-emscripten/clang], [PY_SUPPORT_TIER=3], dnl WebAssembly Emscripten

... should end up producing a warning.