[test/asan] Update a test case to work with old Darwin SDK's

On Darwin, MAP_ANONYMOUS is a synonym for MAP_ANON. However, some SDK's
don't define MAP_ANONYMOUS. Use MAP_ANON to work around this.

(As a point of interest, the situation is exactly reversed on Linux.)

llvm-svn: 267907

compiler-rt/test/asan/TestCases/Darwin/segv_read_write.c

@@ -13,7 +13,7 @@ int main(int argc, char **argv) {
   // shadow mapping works). This is kinda hard to fix. Test a random address in
   // the application part of the address space.
   void *volatile p =
-      mmap(nullptr, 4096, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
+      mmap(nullptr, 4096, PROT_READ, MAP_PRIVATE | MAP_ANON, 0, 0);
   munmap(p, 4096);
   if (argc == 1)
     Read((int *)p);