[asan] Fix-forward undefined type in test from #153142 (#153636)

Fix Mac build breakage (reported by aeubanks in
#153142 (comment))
by including stdint.h and using uintptr_t

Author: thurstond

compiler-rt/test/asan/TestCases/Posix/fakestack_alignment.cpp

@@ -21,6 +21,7 @@
 
 #include <assert.h>
 #include <pthread.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -35,7 +36,7 @@ bool misaligned = false;
 // happen by chance, so try this on many threads.
 void *Thread(void *unused) {
   big_object x;
-  uint alignment = (unsigned long)&x % alignof(big_object);
+  uintptr_t alignment = (uintptr_t)&x % alignof(big_object);
 
   if (alignment != 0)
     misaligned = true;