[release/11.0-preview2] Revert forwarding activation signal (#124894)

Backport of #124877 to release/11.0-preview2

/cc @janvorli

## Customer Impact

- [x] Customer reported
- [ ] Found internally

The current CheckActivationSafePoint uses thread local storage to
get the current Thread instance. But this function is called from
async signal handler (the activation signal handler) and it is not
allowed to access TLS variables there because the access can allocate
and if the interrupted code was running in an allocation code, it
could crash.
There was no problem with this since .NET 1.0, but a change in the
recent glibc version has broken this. We've got reports of crashes
in this code e.g on recent Ubuntu 25.04 due to this issue.

## Regression

- [ ] Yes
- [x] No

## Testing
CI tests, local manual directed tests

## Risk
Low, the change was in main for the past two months and we haven't seen
any issues related to it. The newly added code is executed very
frequently.

Co-authored-by: Jan Vorlicek <janvorli@microsoft.com>

Author: github-actions[bot]

src/coreclr/pal/src/exception/signal.cpp

@@ -965,20 +965,22 @@ static void inject_activation_handler(int code, siginfo_t *siginfo, void *contex
             CONTEXTToNativeContext(&winContext, ucontext);
         }
     }
-
-    // Call the original handler when it is not ignored or default (terminate).
-    if (g_previous_activation.sa_flags & SA_SIGINFO)
-    {
-        _ASSERTE(g_previous_activation.sa_sigaction != NULL);
-        g_previous_activation.sa_sigaction(code, siginfo, context);
-    }
     else
     {
-        if (g_previous_activation.sa_handler != SIG_IGN &&
-            g_previous_activation.sa_handler != SIG_DFL)
+        // Call the original handler when it is not ignored or default (terminate).
+        if (g_previous_activation.sa_flags & SA_SIGINFO)
+        {
+            _ASSERTE(g_previous_activation.sa_sigaction != NULL);
+            g_previous_activation.sa_sigaction(code, siginfo, context);
+        }
+        else
         {
-            _ASSERTE(g_previous_activation.sa_handler != NULL);
-            g_previous_activation.sa_handler(code);
+            if (g_previous_activation.sa_handler != SIG_IGN &&
+                g_previous_activation.sa_handler != SIG_DFL)
+            {
+                _ASSERTE(g_previous_activation.sa_handler != NULL);
+                g_previous_activation.sa_handler(code);
+            }
         }
     }
 }