Auto merge of #115260 - scottmcm:not-quite-so-cold, r=WaffleLapkin

Use `preserve_mostcc` for `extern "rust-cold"`

As experimentation in #115242 has shown looks better than `coldcc`.  Notably, clang exposes `preserve_most` (https://clang.llvm.org/docs/AttributeReference.html#preserve-most) but not `cold`, so this change should put us on a better-supported path.

And *don't* use a different convention for cold on Windows, because that actually ends up making things worse. (See comment in the code.)

cc tracking issue #97544

Author: bors