Latest wasi-libc

[csegarragonz]

In this PR I rebase our wasi-libc fork to the latest upstream. Most importantly, many things that were not supported when we first forked wasi-libc have now been implemented (i.e. emulated). For example, signals or process clocks. So I use those when possible.

The diff between faasm's branch and the latest upstream head can be inspected here. I would like to simplify it further (e.g. removing syscalls support in Faasm?), but this is out of the scope of this PR.

The summary of changes after the rebase are the following:

• Add the option to nuke the sysroot when re-building wasi-libc. Not doing so, prevented catching some bugs of dependencies building with older sysroots.
• mmap/munmap is also supported as part of wasi-libc now. I ammend our demo functions to compile with both our mmap implementation and wasi-libc's.

[Shillaker]

Tiny comment on LLVM version, otherwise LGTM 👍

[Shillaker]

This image was tagged with the LLVM image used to build it (i.e. used to be LLVM 13.0.1). It makes it clearer which LLVM version is being used IMO

[csegarragonz]

When updating the PR description, I removed the explanation of this change.

Turns out that installing LLVM (inc. libc) populates the sysroot with files (e.g. header files) that are not necessarily removed when re-building libc from scratch. Thus, different libc configurations for the same llvm version would yield different container images.

Even though adding the --purge flag to llvm.libcshould do the trick, I am not enitrely sure. Thus, rather than having docker image inconsitencies I change the tagging mechanism.