There's a merged pull request on the void-packages repository

gcc: update to 9.1.0
https://github.com/void-linux/void-packages/pull/12700
Description: Remove `libmpx{,-devel}` as they are no longer supported.

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

Closes: #11663

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