Fix over read in strcpy/stpcpy/strcat.

This bug will happen when these circumstances are met:

- Destination address & 0x7 == 1, strlen of src is 11, 12, 13.
- Destination address & 0x7 == 2, strlen of src is 10, 11, 12.
- Destination address & 0x7 == 3, strlen of src is 9, 10, 11.
- Destination address & 0x7 == 4, strlen of src is 8, 9, 10.

In these cases, the dest alignment code does a ldr which reads 4 bytes,
and it will read past the end of the source. In most cases, this is
probably benign, but if this crosses into a new page it could cause a
crash.

Fix the labels in the cortex-a9 strcat.

Modify the overread test to vary the dst alignment to expost this bug.
Also, shrink the strcat/strlcat overread cases since the dst alignment
variation increases the runtime too much.

Bug: 24345899
Change-Id: Ib34a559bfcebd89861985b29cae6c1e47b5b5855

tests/buffer_tests.cpp

@@ -381,15 +381,19 @@ void RunSrcDstBufferOverreadTest(void (*test_func)(uint8_t*, uint8_t*, size_t))
   // Make the second page unreadable and unwritable.
   ASSERT_TRUE(mprotect(&memory[pagesize], pagesize, PROT_NONE) == 0);
 
-  uint8_t* dst = new uint8_t[pagesize];
-  for (size_t i = 0; i < pagesize; i++) {
-    uint8_t* src = &memory[pagesize-i];
+  uint8_t* dst_buffer = new uint8_t[2*pagesize];
+  // Change the dst alignment as we change the source.
+  for (size_t i = 0; i < 16; i++) {
+    uint8_t* dst = &dst_buffer[i];
+    for (size_t j = 0; j < pagesize; j++) {
+      uint8_t* src = &memory[pagesize-j];
 
-    test_func(src, dst, i);
+      test_func(src, dst, j);
+    }
   }
   ASSERT_TRUE(mprotect(&memory[pagesize], pagesize, PROT_READ | PROT_WRITE) == 0);
   free(memory);
-  delete[] dst;
+  delete[] dst_buffer;
 }
 
 void RunCmpBufferOverreadTest(