[llvm] Fix signal handling for platforms without SA_ONSTACK (PR #118962)

[Valery Chernov via llvm-commits]

vvchernov wrote:

> Should this need a configure-time check / define (check_symbol_exists or something)?

Isn't the current implementation already enough? If there is a better mechanism for that kind of check (but I do not know such),  then it would be good to use it. Could you show an example if the mechanism exists?

https://github.com/llvm/llvm-project/pull/118962