There is an updated pull request by pullmoll against master on the void-packages repository

https://github.com/pullmoll/void-packages gcc91
https://github.com/void-linux/void-packages/pull/12700

gcc: update to 9.1.0
Remove `libmtx{,-devel}` as they are no longer supported.
No package used them thus not sure if we need a replaces="libmtx>=0" etc.

No longer try to build `gcc-gdc` as cross compiling fails and I don't feel like wasting more time on it.

Closes: #11663

[ci skip]

A patch file from https://github.com/void-linux/void-packages/pull/12700.patch is attached