Probably safer to check all of them instead of just CC, since previously
it seems the loongarch64 RANLIB variable could have been wrong.
I am not sure if a missing binary could cause unexpected behaviour.
It seems `g++-loongarch64-linux-gnu` does not exist in the Ubuntu
universe repositories. It also fails using the ubuntu Docker image
locally (with the universe repo added).
`g++-14-loongarch-linux-gnu` exists on both Ubuntu 22.04 and 24.04,
however ideally we want to use whatever works on the most platforms.
Since `scripts/test` always unset "CC", the github action for "clang"
in Linux does not use clang unintentionally.
This patch unset CC only for mingw{32,64} and arm32.
This reworks the macOS github actions to target Apple silicon for tests.
To simplify the test matrix, removing big sur and adding ventura.
Also some cleanups to the test script. We really don't need the 'x'
workaround for non-POSIX shells, and it got confusing writing "xx86_64".
