[msan] Get stack limits with pthread_create interceptor.

Before we did it lazily on the first stack unwind in the thread.
It resulted in deadlock when the unwind was caused by memory allocation
inside pthread_getattr_np:
  pthread_getattr_np   <<< not reentable
  GetThreadStackTopAndBottom
  __interceptor_realloc
  pthread_getattr_np
  

llvm-svn: 197026
diff --git a/compiler-rt/lib/msan/msan.h b/compiler-rt/lib/msan/msan.h
index 4e6c619..a50540b 100644
--- a/compiler-rt/lib/msan/msan.h
+++ b/compiler-rt/lib/msan/msan.h
@@ -106,4 +106,10 @@
 #define MSAN_FREE_HOOK(ptr) \
   if (&__msan_free_hook) __msan_free_hook(ptr)
 
+struct MsanStackBounds {
+  uptr stack_top, stack_bottom;
+};
+
+extern THREADLOCAL MsanStackBounds msan_stack_bounds;
+
 #endif  // MSAN_H