[compiler-rt] [ubsan] Support static linking with standalone runtime (PR #80943)

[Vitaly Buka via llvm-commits]

================
@@ -66,6 +66,11 @@ void InitializeDeadlySignals() {
     return;
   is_initialized = true;
   InitializeSignalInterceptors();
+#if SANITIZER_INTERCEPT_SIGNAL_AND_SIGACTION
+  // REAL(sigaction_symname) is nullptr in a static link. Bail out.
+  if (!REAL(sigaction_symname))
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vitalybuka wrote:

Actually signal interceptors are there :(

I think the best for `-static` is to avoid code
```
#define SIGNAL_INTERCEPTOR_ENTER() __ubsan::InitializeDeadlySignals()
#include "sanitizer_common/sanitizer_signal_interceptors.inc"
```

maybe in any  UBSAN

`InitializeSignalInterceptors` is only needed to enforce options like `handle_segv=2`
which are not very critical

https://github.com/llvm/llvm-project/pull/80943