Don't use _BitScanForward64/_BitScanReverse64 on 32-bit MSVC builds

677319746 added support for making use of MSVC's bit scanning functions.
However, that commit failed to consider 32-bit MSVC builds where the
64-bit versions of these functions are unavailable.  This resulted in
compilation failures on 32-bit MSVC.

Here we adjust the code so we fall back on the manual way of finding the
bit positions for 64-bit integers when building on 32-bit MSVC.

Bug: #17967
Reported-by: Youmiu Mo
Discussion: https://postgr.es/m/17967-cd21e34a314141b2@postgresql.org

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