os: fix Chtimes timestamp overflow for dates outside 1677-2262

[zheliu2]

os.Chtimes used time.Time.UnixNano() to convert timestamps to syscall
timespecs, which overflows int64 for dates before ~1677 or after ~2262.
This corrupts timestamps on filesystems that support wider date ranges
(ext4, NTFS, APFS, etc.).

Changes:

• Add syscall.SecNsecToTimespec(sec, nsec int64) that takes separate
  seconds and nanoseconds, avoiding the seconds * 1e9 overflow inherent
  in NsecToTimespec
• Use SecNsecToTimespec(t.Unix(), int64(t.Nanosecond())) in
  os.chtimesUtimes instead of NsecToTimespec(t.UnixNano())
• Fix the same overflow in Windows codepaths (syscall.UtimesNano and
  os.chtimesat) by computing FILETIME ticks from seconds and
  nanoseconds separately
• Add TestChtimesExtremeDates covering years 1000, 1600, 2300, and 3000

Fixes #75542

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 2:

(1 comment)

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