But Rust does support S390x?
>Worse still, Rust seems to basically leave anything that isn't i686/x86_64 or ARM64 as "Tier 2" or worse
Rust has an explicit documented support tier list with guarantees laid out for each level of support. Point me to a document where GCC or Clang lists out their own explicit guarantees on a platform-by-platform basis.
Because I strongly suspect that the actual "guarantees" which GCC, clang and so forth provide for most obscure architectures is not that much better than Rust, if at all - just more ambiguous. And I don't find it very likely that the level of quality control for C compilers on m68k or alpha or s390x is not, in practice, at least a bit lower than that provided for x86 and ARM.