Enable mmap for WASI if it supports the mman header

[zebreus]

#1509 completely removed the mmap support when targeting WASI. wasi-libc supports an optional emulated mmap implementation if _WASI_EMULATED_MMAN is set. Emulated mmap calls in wasi-libc come with a few restrictions, but users are aware of them as the feature is opt-in.

This PR enables mmap support for WASI if _WASI_EMULATED_MMAN is defined.

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