Fix failing to create DAG file larger than 2 Gb

[gumb0]

Fixes #4021

fseek has 32-bit offset parameter, and there's no fully-portable way in C to do 64-bit seek, so we resort to different version for Windows & POSIX.

[codecov-io]

Codecov Report

Merging #4023 into develop will decrease coverage by 0.04%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop    #4023      +/-   ##
===========================================
- Coverage    65.05%   65.01%   -0.05%     
===========================================
  Files          306      306              
  Lines        21350    21352       +2     
===========================================
- Hits         13889    13881       -8     
- Misses        7461     7471      +10

Impacted Files	Coverage Δ
libethash/io.h	100% <ø> (ø)	⬆️
libethash/io_posix.c	85.36% <0%> (-4.38%)	⬇️
libethash/io.c	46.34% <0%> (ø)	⬆️
test/tools/fuzzTesting/fuzzHelper.cpp	31.97% <0%> (-4.07%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7cadd20...8209f08. Read the comment docs.