Add OpenBSD support in CPalThread (#129057)

Add OpenBSD support for `CPalThread::GetStackBase()` and
`CPalThread::GetStackLimit()`.

OpenBSD does not have `pthread_attr_get_np` or `pthread_getattr_np` to
get the current thread's stack attributes.

Instead there is a standalone function
[pthread_stackseg_np](https://man.openbsd.org/pthread_stackseg_np.3)
that does the same via [stack_t](https://man.openbsd.org/sigaltstack.2).
Since we do not actually need a standard pthread attr in this case I put
this case inside a `TARGET_OPENBSD` check similar to macOS.

Contributes to #124911

---------

Co-authored-by: Jan Kotas <jkotas@microsoft.com>

Author: sethjackson

src/coreclr/pal/src/thread/thread.cpp

@@ -1423,7 +1423,7 @@ CPalThread::ThreadEntry(
                     // vendor-modified Android kernels with strict SELinux policy) block
                     // sched_setaffinity even when passed a mask extracted via sched_getaffinity.
                     // Treat this as non-fatal — the thread will continue running on any
-                    // available CPU rather than the originally affinitized one. 
+                    // available CPU rather than the originally affinitized one.
                     WARN("sched_setaffinity failed with EPERM/EACCES, ignoring\n");
                 }
                 else
@@ -2316,6 +2316,12 @@ CPalThread::GetStackBase()
 #ifdef TARGET_APPLE
     // This is a Mac specific method
     stackBase = pthread_get_stackaddr_np(pthread_self());
+#elif defined(TARGET_OPENBSD)
+    stack_t stack;
+    int status = pthread_stackseg_np(pthread_self(), &stack);
+    _ASSERT_MSG(status == 0, "pthread_stackseg_np call failed");
+
+    stackBase = stack.ss_sp;
 #else
     pthread_attr_t attr;
     void* stackAddr;
@@ -2361,6 +2367,12 @@ CPalThread::GetStackLimit()
     // This is a Mac specific method
     stackLimit = ((BYTE *)pthread_get_stackaddr_np(pthread_self()) -
                    pthread_get_stacksize_np(pthread_self()));
+#elif defined(TARGET_OPENBSD)
+    stack_t stack;
+    int status = pthread_stackseg_np(pthread_self(), &stack);
+    _ASSERT_MSG(status == 0, "pthread_stackseg_np call failed");
+
+    stackLimit = (void*)stack.ss_size;
 #else
     pthread_attr_t attr;
     size_t stackSize;