On x64 hardware, round-trip uint64_t→double→uint64_t conversions

On x64 hardware, round-trip uint64_t→double→uint64_t conversions
fail for values greater than UINT64_MAX-2047.  This caused the SQLite
text-to-float converter routine to give incorrect results for values
between '1.8446744073709550592eNNN' and '1.8446744073709551609eNNN' for any
exponent NNN.  This problem was introduced by check-in [761d8fd18b0ee868]
and first appeared in version 3.47.0 and was reported by
[forum:/forumpost/569a7209179a7f5e|forum post 569a7209179a7f5e].  Fixed
by this check-in.

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