[esan] Use stack_t instead of struct sigaltstack (PR34011)

The struct tag is going away in soon-to-be-released glibc 2.26 and the
stack_t typedef seems to have been there forever.

Patch by Bernhard Rosenkraenzer!

llvm-svn: 311495

compiler-rt/lib/esan/esan_sideline_linux.cpp

@@ -70,7 +70,7 @@ int SidelineThread::runSideline(void *Arg) {
 
   // Set up a signal handler on an alternate stack for safety.
   InternalScopedBuffer<char> StackMap(SigAltStackSize);
-  struct sigaltstack SigAltStack;
+  stack_t SigAltStack;
   SigAltStack.ss_sp = StackMap.data();
   SigAltStack.ss_size = SigAltStackSize;
   SigAltStack.ss_flags = 0;