[compiler-rt] Reland '[hwasan] Add fixed_shadow_base flag (#73980)' (PR #95445)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

> If you can wait, I can confirm this works later today by cherry-picking this and trying to reproduce the Android problem.

LGTM but not critical, as the cause looks straightforward.

However fix for flag initialization will need real testing.

https://github.com/llvm/llvm-project/pull/95445