initial-exec not reduced to less strict hint on unsupported platforms

[Mark-Simulacrum]

As discovered in #81334 and "fixed" for the compiler in #85807, -Ztls-model=initial-exec does not work on some platforms. However, it seems like LLVM and/or rustc can automatically detect this on known unsupported platforms and either ignore or reduce the hint as needed.

See #85807 (comment), #85807 (comment).