Turn pause_and_exit() into pause_forever()

Summary:
pthread_exit can lead to confusing crashes for us: it raises
a "forced unwind", which it seems (strangely) will enter into C++
catch (...) blocks: but if you don't rethrow from one these blocks,
glibc will call abort() from a function called unwind_cleanup.  Our
LibEventWorker has a catch block of this sort that doesn't rethrow, so
calling pthread_exit below there will abort the program.  (The point
of this function is to just wait while another thread writes a
stacktrace file, so there's no reason to exit the thread.)

src/runtime/base/builtin_functions.cpp

@@ -1000,14 +1000,13 @@ Object create_object(CStrRef s, CArrRef params, bool init /* = true */,
   }
 }
 
-void pause_and_exit() {
-  // NOTE: This is marked as __attribute__((noreturn)) in base/types.h
-  // Signal sent, nothing can be trusted, don't do anything, as we might
-  // write bad data, including calling exit handlers or destructors until the
-  // signal handler (StackTrace) has had a chance to exit.
-  sleep(300);
-  // Should abort first, but it not try to exit
-  pthread_exit(0);
+/*
+ * This function is used when another thread is segfaulting---we just
+ * want to wait forever to give it a chance to write a stacktrace file
+ * (and maybe a core file).
+ */
+void pause_forever() {
+  for (;;) sleep(300);
 }
 
 void check_request_surprise(ThreadInfo *info) {

src/runtime/base/types.h

@@ -397,21 +397,21 @@ inline void check_recursion(ThreadInfo *&info) {
 }
 
 // implemented in runtime/base/builtin_functions.cpp
-extern void pause_and_exit() ATTRIBUTE_COLD ATTRIBUTE_NORETURN;
+extern void pause_forever() ATTRIBUTE_COLD ATTRIBUTE_NORETURN;
 extern void check_request_surprise(ThreadInfo *info) ATTRIBUTE_COLD;
 
 extern bool SegFaulting;
 
 inline void check_request_timeout(ThreadInfo *info) {
   const_assert(!hhvm);
-  if (SegFaulting) pause_and_exit();
+  if (SegFaulting) pause_forever();
   info->m_mm->refreshStats();
   if (info->m_reqInjectionData.conditionFlags) check_request_surprise(info);
 }
 
 inline void check_request_timeout_nomemcheck(ThreadInfo *info) {
   const_assert(!hhvm);
-  if (SegFaulting) pause_and_exit();
+  if (SegFaulting) pause_forever();
   if (info->m_reqInjectionData.conditionFlags) check_request_surprise(info);
 }