MemoryBuffer.cpp: Consider if PageSize were not 4096 in shouldUseMmap(). Follow-up to r188903.

The AllocationGranularity can be 65536 on Win32, even on Cygwin.
MemoryBufferTest.cpp: Tweak offset corresponding to the case that PageSize is greater than 8000.

PageSize, aka AllocationGranularity, is 65536 on Win32 (and Cygwin).