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

 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