[sanitizer] Fix build with _FILE_OFFSET_BITS=64.

Sanitizer source is not affected by _FILE_OFFSET_BITS in general,
but this one file must be built with 32-bit off_t. More details in the code.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@220328 91177308-0d34-0410-b5e6-96231b3b80d8