Windows DLL base addresses must be adjusted for 64-bit

With recent MSVC versions (e.g. 15.6.2) we get warnings when building a 64-bit Qt:

LNK4281 - "undesirable base address 0x6400000 for x64 image; set base address above 4GB for best ASLR optimization".

For 64-bit builds we must either

• specify higher base addresses
• or don't specify the base address at all

The former potentially reduces DLL load times and memory footprint [1].

The latter is recommended by Microsoft for security reasons [2].

[1] Rebasing Win32 DLLs
[2] /BASE (Base Address)