[asan] use correct type when computing shadow address in allocator (caused SEGV when asan run-time is built with gcc 4.8.2 on 32-bit arch)

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@199544 91177308-0d34-0410-b5e6-96231b3b80d8