innodb_buffer_pool_size >= 4096M on x64 Windows

All three srv_buf_pool related variables are defined as ulong, which is
32-bit on 64-bit Windows. They are changed to 64-bit ulint. Also
system_info.dwPageSize appears to be 32-bit only. Casting to 64-bit
is required.

Approved by:	Marko (on IM)