Update the IRC link to the link of the Matrix channel

Author: antoyo

Readme.md

@@ -1,6 +1,6 @@
 # WIP libgccjit codegen backend for rust
 
-[![Chat on IRC](https://img.shields.io/badge/irc.libera.chat-%23rustc__codegen__gcc-blue.svg)](https://web.libera.chat/#rustc_codegen_gcc)
+[![Chat on IRC](https://img.shields.io/badge/matrix.org-%23rustc__codegen__gcc-blue.svg)](https://matrix.to/#/#rustc_codegen_gcc:matrix.org)
 
 This is a GCC codegen for rustc, which means it can be loaded by the existing rustc frontend, but benefits from GCC: more architectures are supported and GCC's optimizations are used.