[clang] [compiler-rt] [safestack] Various Solaris fixes (PR #98001)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> At the end of the recent flurry of Illumos (and implicitly Solaris) safestack patches, the tests were disabled in
> [b0260c5](https://github.com/llvm/llvm-project/commit/b0260c5b1052f8e3ff1ec77dc42a11f42da762cc) without explanation.

@devnexen  Thank you for #97183, which seemed to addressed a real issue, but make sure to include descriptions in the future.

https://github.com/llvm/llvm-project/pull/98001