Make bazel toolchain more usable

[bianpengyuan]

Right now bazel toolchain is not very user friendly since

• It requires copy all tool chain files, which is hard to maintain.
• It introduces bazel dependencies from rule_nodejs, which is unnecessary for cpp target.
• It hard codes to use linux binary, which will break on other platforms.

[walkingeyerobot]

Thanks for the feedback! I'm happy to look into this, but I could use a few more details.

It requires copy all tool chain files, which is hard to maintain.

1. Bazel has to get a copy of emscripten from somewhere. I've found this is pretty easy to set up in clean environments. What would you prefer here?

It introduces bazel dependencies from rule_nodejs, which is unnecessary for cpp target.

2. As Emscripten uses nodejs and node modules, depending on the bazel nodejs rules seems like a reasonable dependency. Are you having trouble with using the toolchain as a result? Can you elaborate more on why you dislike this dependency?

It hard codes to use linux binary, which will break on other platforms.

3. This is clearly a deficiency. I agree it should be fixed. I'll work on adding mac / windows support.