[sanitizer] Adjust max read size in FlagHandlerInclude

Setting the maximum read size in FlagHandlerInclude to 2^15 might be a good
default, but causes the read to fail on systems with a page size larger than
that (ReadFileToBuffer(...) will fail if the maximum allowed size is less than
the value returned by GetPageSizeCached()). For example, on my PPC64/Linux
system, GetPageSizeCached() returns 2^16. In case the page size is larger, use
that instead.

llvm-svn: 226368