Update WASI platform support

[whitequark]

This is essentially a part of #144 that I've missed because I tested against an outdated version of Boost at that time. See that PR for details.

[Lastique]

If there is no access mode in WASI, then I'd prefer to also disable adding S_IWUSR above, along with its associated comment.

[whitequark]

Something like this?

[Lastique]

More like this:

mode_t to_mode = from_mode;
#if !defined(__wasm)
...
#endif

[Lastique]

Merged to develop.

[whitequark]

Thanks!