elf: apply R_X86_64_RELATIVE addend semantics

[jlinenkohl]

Summary

• Correct R_X86_64_RELATIVE relocation behavior to apply base + addend semantics.

Why

• Required for correct dynamic loader/bootstrap relocation behavior.

Validation

• cd tests && bash run_unit_tests.sh -R test_elf

Depends on

• linux: fix readlinkat guest copy length handling #67 (recommended)

Stack Context

• Part of relocation runtime stack.
• Predecessor for elf: add .relr.dyn bootstrap relocation support #69.

Test Evidence

• Date: 2026-04-02
• Branch-level validation source: phase14_audit baseline sweep
• Full unit harness: cd tests && bash run_unit_tests.sh -> 8/8 passed
• Integration tinytest lane: (cd guest/tests && bash build.sh) && ./build/tinytest guest/tests/glibc_test -> passed

PR-Scoped Command

• cd tests && bash run_unit_tests.sh -R test_elf
• Stacked expectation: evaluate with linux: fix readlinkat guest copy length handling #67; may be partial if reviewed in isolation

[fwsGonzo]

Thanks!