[sanitizer] increase the mmap granularity in sanitizer allocator from 2^16 to 2^18. This is a partial workaround for the fresh Kernel bug https://bugzilla.kernel.org/show_bug.cgi?id=67651

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200197 91177308-0d34-0410-b5e6-96231b3b80d8