Add emulated features from wasi-libc (like mmap)

[GregoryConrad]

When compiling to wasm32-wasi, libc does not include some emulated wasi-libc functions like mmap. When using wasi-libc directly, you can set some extra parameters to enable these emulated functions like:

-D_WASI_EMULATED_MMAN   # for clang
-lwasi-emulated-mman    # for lld

What would be the appropriate fix for this? I was hoping a cargo feature or something similar could solve the problem. I am a bit new here, so if I could get some guidance I would be happy to PR to supply these additions. (I see that changes would be needed for https://github.com/rust-lang/libc/blob/master/src/wasi.rs, but I am not sure what would need to change in build.rs or elsewhere to accommodate this.)

[GregoryConrad]

I think I got a bit confused; I thought this libc was bundling the target libc implementation itself but it looks as though it isn't. It just handles the ffi bindings (please do correct me if I am wrong).

I'll make a PR with a cargo feature for wasi-emulated-mman (or something else if someone has a better suggestion) with the associated bindings, so that people building with their own wasi-libc sysroot can turn on the cargo feature and use mman.h.

[GregoryConrad]

See #3093 for why this is getting closed.