hermit: make stat::st_size signed

[mkroening]

Description

This PR changes Hermit's stat::st_size from u64 to i64.

Sources

This was done upstream in hermit-os/kernel#1576 to comply with POSIX.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

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