port: mmap offset alignment on Windows

On Windows the mmap offset is not aligned to the page size but to the
allocator granularity which is usually bigger than the page size. Add a new
internal function to the mmap compat code to get the proper offset alignment
size for each platform.

See the "dwFileOffsetLow" section for more details :
https://docs.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-mapviewoffile

Change-Id: Ic26dc53862719892738a88b3dcea6a15901a0c94
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2107
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>