[WinASan] Fix page size and mmap granularity confusion

We were erroneously reporting 16K as the page size on Windows because
the code that does the shadow mapping was using page size instead of
allocation granularity. After fixing that, we can resolve the FIXMEs in
the Windows implementations of GetPageSize and GetMmapGranularity by
calling GetSystemInfo instead of returning hard-coded, incorrect
answers.

llvm-svn: 261233