Add MapFlags::MAP_UNINITIALIZED for Linux

[raymontag]

Linux supports the map flag MAP_UNINITIALIZED for embedded devices. Please add this for the MapFlags type.

https://elixir.bootlin.com/linux/latest/source/include/uapi/asm-generic/mman-common.h#L33

[asomers]

Care to submit a PR?

[raymontag]

I created a PR for rust-libc: rust-lang/libc#3274

If this is accepted, I'll create my PR for nix