[safestack] Don't crash if stack size is not aligned as expected

[vitalybuka]

Summary

From runtime side looks it's OK to RoundUpTo to needed alignment as buffer is
going to be RoundUpTo to page size anyway.

[eugenis]

FYI @vlad.tsyrklevich
LGTM

compiler-rt/trunk/lib/safestack/safestack.cc

if (attr) {
     pthread_attr_t tmpattr;
     pthread_attr_init(&tmpattr);
     pthread_attr_getstacksize(&tmpattr, &size);
     pthread_attr_getguardsize(&tmpattr, &guard);
     pthread_attr_destroy(&tmpattr);
   }
 
   SFS_CHECK(size);
-  SFS_CHECK((size & (kStackAlign - 1)) == 0);
+  size = RoundUpTo(size, kStackAlign);
   SFS_CHECK((guard & (pageSize - 1)) == 0);
 
   void *addr = unsafe_stack_alloc(size, guard);
   struct tinfo *tinfo =
       (struct tinfo *)(((char *)addr) + size - sizeof(struct tinfo));
   tinfo->start_routine = start_routine;
   tinfo->start_routine_arg = arg;
   tinfo->unsafe_stack_start = addr;