Add support for HermitOS

[notgull]

HermitOS is a microkernel target aiming to provide a simple OS for
virtualized applications. It recently added support for the poll() and
eventfd() system calls, which means we can target it with our poll()
based backend.

Hermit does not have a traditional libc; instead it uses hermit-abi.
However rustix does not support using hermit-abi as its underlying
layer yet. So we have to build a shim layer until it does.

Closes #177
cc bytecodealliance/rustix#1012
cc @stlankes

[stlankes]

Looks good from my side! However, we have an issue last_os_error() and it doesn't work like expected. But we are able to fix it. I will give you a hit, it it works like expected.

[notgull]

Looks good from my side! However, we have an issue last_os_error() and it doesn't work like expected. But we are able to fix it. I will give you a hit, it it works like expected.

Could you elaborate? What error is being returned?

[stlankes]

In std we determine the last error code in a different way and ignored last_os_error. We just don't use it. To solve it, we submit rust-lang/rust#121765. In the current stage, I would directly use hermit-abi to get the last error code.

[notgull]

@stlankes I've fixed it to use get_errno instead of last_os_error, can you take another look?