Add wasi support

[quink-black]

WASI is WebAssembly System Interface. Now we can build ncnn with wasi-sdk[1] to create a WebAssembly binary module, and run it with wasmtime on cmdline.

[1] https://github.com/WebAssembly/wasi-sdk [2] https://github.com/bytecodealliance/wasmtime

[tencent-adm]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.65%. Comparing base (4c3deba) to head (a3e9b8d). Report is 4 commits behind head on master.

:exclamation: Current head a3e9b8d differs from pull request most recent head decc508

Please upload reports for the commit decc508 to get more accurate results.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #5534       +/-   ##
===========================================
- Coverage   94.66%   92.65%    -2.02%     
===========================================
  Files         787      368      -419     
  Lines      248712    91067   -157645     
===========================================
- Hits       235432    84374   -151058     
+ Misses      13280     6693     -6587     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[nihui]

Thanks for your contribution !