CMake: MSVC 64-bit cleanup

[dg0yt]

What does this PR do?

This PR revises CMake code for MSVC.

• Variable CMAKE_CL_64 is discouraged:
  https://cmake.org/cmake/help/latest/variable/CMAKE_CL_64.html
• Macro _WIN64 is defined by MSVC preprocessor:
  https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-170
• /machine:x64 breaks arm64.
  (Ported from microsoft/vcpkg@dcf8a70#diff-d9dc7eb8a09a06ac0d7d8954d0422637dd33206bd73bee02952f1222f9438630R9.)

Tasklist

• Review
• Adjust for comments
• All CI builds and checks have passed

Environment

Provide environment details, if relevant:

• OS: Windows
• Compiler: MSVC

---

No local testing due to no MSVC.

[dg0yt]

Removed FindDB2.cmake.
There are interesting hits for git grep DB2 (frmts/drivers.ini, and HACK_FOR_IBM_DB2_V72), but this out of scope for this PR.

[rouault]

frmts/drivers.ini,

just removed that one

HACK_FOR_IBM_DB2_V72

hum, that one dates back to times where I was hardly born. It doesn't related with the DB2 driver that was added (and eventually removed) after this HACK_FOR_IBM_DB2_V72