AK: Use getrlimit() to find the correct main thread stack size on macOS

This is what JavaScriptCore does as well.

(cherry picked from commit 11458f0d91654feed9b4e225c4e6ac8528057355)

AK/StackInfo.cpp

@@ -65,11 +65,16 @@ StackInfo::StackInfo()
     // MacOS seems inconsistent on what stack size is given for the main thread.
     // According to the Apple docs, default for main thread is 8MB, and default for
     // other threads is 512KB
-    constexpr size_t eight_megabytes = 0x800000;
-    if (pthread_main_np() == 1 && m_size < eight_megabytes) {
-        // Assume no one messed with stack size linker options for the main thread,
-        // and just set it to 8MB.
-        m_size = eight_megabytes;
+    if (pthread_main_np() == 1) {
+        // Apparently the main thread's stack size is not reported correctly on macOS
+        // but we can use getrlimit to get the correct value.
+        rlimit limit {};
+        getrlimit(RLIMIT_STACK, &limit);
+        if (limit.rlim_cur == RLIM_INFINITY) {
+            m_size = 8 * MiB;
+        } else {
+            m_size = limit.rlim_cur;
+        }
     }
     m_base = top_of_stack - m_size;
 #elif defined(AK_OS_OPENBSD)