build: use `_fseeki64()` on Windows, drop detections

build: use `_fseeki64()` on Windows, drop detections

A recent update caused CMake builds to mis-detect this symbol on iOS.
Auto-detection also seems redundant given that it's a Windows-only
function and most Windows builds were already opted-in.

Drop detections and use it in all Windows builds with large file support
enabled.

Feature history:
- pririotizing for Windows: aaacd02466f77d03b8fdc19e91a0a3ec72f4c38a #14678
- Windows opt-in cmake: 8e74c0729d0cace00a202fc6c33c1b35703e220a #11950
- Windows opt-in: aa6c94c5bf4f5caa31c0213d9cd7058c29a9b30b #11944
- use in libcurl: 9c7165e96a3a9a2d0b7059c87c699b5ca8cdae93 #11918
- use in example: 817d1c01064ac81e9609819b15738ee540ef056c

Regression from 8e345057761a8f796403923a96f2c8fd3edca647 #15164

Reported-by: Maarten Billemont
Fixes #15525
Closes #15526