Support WASM/Emscripten

[justsmth]

Issues:

Addresses: P372521248

Description of changes:

This PR adds support for building and testing AWS-LC with WebAssembly (WASM) using an Emscripten toolchain.

Description

• Defines OPENSSL_WASM when compiling with __wasm__ to enable WASM-specific code paths
• Configures WASM builds to use getentropy() for random number generation, which Emscripten provides
• Provide CMake toolchain file: util/wasm-toolchain.cmake
• Added tests/ci/run_emscripten_tests.sh for testing.
• Added CI workflow: .github/workflows/emscripten.yml

Call-outs:

• Threading is enabled via Emscripten's pthread implementation using Web Workers and SharedArrayBuffer
• Tests that require fork() or POSIX sockets are excluded as these are not supported in the WASM/Emscripten environments

Testing:

• The new CI workflow builds AWS-LC and runs crypto_test, urandom_test, and ssl_test in a WASM/Node.js environment
• All tests pass except those explicitly filtered out due to WASM platform limitations (fork/socket operations)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.32%. Comparing base (ab1b674) to head (2260987).
⚠️ Report is 43 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2959      +/-   ##
==========================================
+ Coverage   78.31%   78.32%   +0.01%     
==========================================
  Files         689      689              
  Lines      121007   121007              
  Branches    16996    16972      -24     
==========================================
+ Hits        94765    94783      +18     
+ Misses      25346    25329      -17     
+ Partials      896      895       -1     

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[sgmenda]

lgtm, but I think we should add a paragraph under "Platform Support" in the repo README.md about security implications.

[sgmenda]

lgtm, I like the README changes