[scudo] Fix MallocIterateBoundary on 32 bit.

[cferris]

Summary

On Android, the 32 bit value of 8 for BlockDelta results in a
failure because a valid pointer can never be found.

Change the code to always use 16 which passes on both 32 bit
and 64 bit.

Verified that re-introducing the old bug causes the test to fail
on both 32 bit and 64 with the new BlockDelta value.

compiler-rt/lib/scudo/standalone/tests/wrappers_c_test.cpp

 }
 
 // Verify that a block located on an iteration boundary is not mis-accounted.
 // To achieve this, we allocate a chunk for which the backing block will be
 // aligned on a page, then run the malloc_iterate on both the pages that the
 // block is a boundary for. It must only be seen once by the callback function.
 TEST(ScudoWrappersCTest, MallocIterateBoundary) {
   const size_t PageSize = sysconf(_SC_PAGESIZE);
-  const size_t BlockDelta = FIRST_32_SECOND_64(8U, 16U);
+  const size_t BlockDelta = 16U;
   const size_t SpecialSize = PageSize - BlockDelta;
 
   // We aren't guaranteed that any size class is exactly a page wide. So we need
   // to keep making allocations until we succeed.
   //
   // With a 16-byte block alignment and 4096-byte page size, each allocation has
   // a probability of (1 - (16/4096)) of failing to meet the alignment
   // requirements, and the probability of failing 65536 times is