compile wams64 using bazel mode about emsdk

[sbc100]

To build for wasm64 you can add -sMEMORY64 to you compile and link commands.

[phoenix20162016]

@sbc100 follows your suggestion，another error occurs, the following is my bazel configuration

in the fold of the compiler tools, the wasm64-emscripten dir really does not exist

[sbc100]

I see, yes we don't currently wasm64 versions of any core libraries in the bazel toolchain. I think this is the same as #807. Perhaps we can close this and continue the discussion there?

[wzheng21]

@phoenix20162016 I had the same problem as yours. For whoever having similar issues, I had to patch emsdk setting FROZEN_CACHE = False in config.py, then the issue becomes read-only blabla error, basically indicating emscripten tries to compile libraries and writes into a temporary cache folder but failed.

This of course would fail, due to bazel's idea of keeping builds hermitian. The following are the two different workarounds I tested that worked:

1. Disable bazel sandbox mode. Just specify --spawn_strategy=local. Be careful when using this, especially if this is not a toy project
2. Don't play with sandbox, but specify --action_env=EM_CACHE=~/.cache/emscripten_cache. When FROZEN_CACHE is False, emscripten will try to write to ~/.emscripten_cache, which will definitely fail in bazel sandbox mode due to permissions. Writing to ~/.cache folder, however, is allowed.

Hope this is helpful for anyone getting stuck with this issue.