Compiler builtins should now be downloaded from crates.io

[HadrienG2]

Now that rust-lang/rust#56092 has landed, and that the nightlies are fixed, cargo-xbuild fails because it cannot find the compiler builtins source in rust-src. This is normal: it is not a submodule anymore, and must now be downloaded from crates.io. To quote @alexcrichton in that PR...

As a proof of concept, this commit removes the dlmalloc,
libcompiler_builtins, and libc submodules from this repository. Long
thorns in our side these are now gone for good and we can directly
depend on crates.io! It's hoped that in the long term we can bring in
other crates as necessary, but for now this is largely intended to
simply make it easier to manage these crates and remove submodules.

This should be a transparent non-breaking change for all users, but one
possible stickler is that this almost for sure breaks out-of-tree
std-building tools like xargo and cargo-xbuild. I think it should
be relatively easy to get them working, however, as all that's needed is
an entry in the [patch] section used to build the standard library.
Hopefully we can work with these tools to solve this problem!

The PR description contains more details about what exactly has changed, and how cargo-xbuild should be modified to account for this change.

[HadrienG2]

Relevant patch for xargo: japaric#229 .
Or you can use this one if you prefer: japaric#228 .

[phil-opp]

Thanks for reporting. I'm looking into this now