GH-49952: [C++][Gandiva] Use timegm in date_time_test utilities

[Reranko05]

Rationale for this change

As discussed in PR #49887, the Gandiva datetime test utilities currently use mktime(), which interprets timestamps as local time and can behave differently across platforms and DST boundaries.

This PR switches the test utilities to timegm() so timestamps are interpreted consistently as UTC. This also removes the existing workaround logic in Epoch() that was previously needed for mktime() behavior on Windows/MSVC.

What changes are included in this PR?

• replaced mktime() with timegm() in date_time_test.cc
• added a small Windows compatibility mapping for _mkgmtime
• removed the existing DST/localtime workaround logic in Epoch()

Are these changes tested?

• Re-ran DateTimeTestProjector.TestFromUtcTimestamp repeatedly locally
• Re-ran the full gandiva-projector-test suite successfully

Are there any user-facing changes?

No.

Github Issue: #49952

• GitHub Issue: [C++][Gandiva] Use timegm in date_time_test utilities to avoid DST-dependent behavior #49952

[github-actions]

⚠️ GitHub issue #49952 has been automatically assigned in GitHub to PR creator.

[Reranko05]

@lriggs I tried the timegm() approach suggested in #49887 locally and opened this follow-up PR based on it.