fs: use ftruncate on OpenBSD in AllocateFileRange #36189

Summary

OpenBSD provides neither fallocate nor posix_fallocate, so AllocateFileRange() always fell through to the slow fallback path there: writing the entire range one 64KiB buffer at a time instead of just advising the OS of the eventual file size.

This adds an __OpenBSD__ branch that uses ftruncate, matching the guidance in #32643 and the same pattern already used for the macOS branch just above it in this function.

Fixes #32643

Test plan

I don't have OpenBSD hardware to run this on directly, so it's verified by:

• The "OpenBSD Cross" CI job, which compiles this exact branch for the target and passes.
• Full local build (macOS, Clang) with no new warnings.
• Full unit test suite (test_bitcoin, 825 cases) passes.
• test/lint/lint-files.py, lint-includes.py, and lint-locale-dependence.py all pass.

The approach mirrors a previous attempt at this issue (#32645, closed unmerged) which was hand-tested on real OpenBSD 7.7 via a signet IBD run, confirming the fallback path is no longer hit.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36189.

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Reviews

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Please don't post LLM PR descriptions. That "Test Plan" doesn't tell anything besides the bare minimum required to make CI pass.

Ready for review: @theStack @fanquake