AOT: skip zero-byte helper-runtime unit.o — hash_hmac_algos link (#6229)

[PurHur]

Summary

• HelperRuntimeCache::unitObjectIsLinkable() rejects unit.o files with filesize() === 0.
• helperIndex() and linkObjects() use it so interrupted emit artifacts do not shadow committed prelinked helper objects.
• Fixes AOT link failure: undefined reference to PHPCompiler_ext_standard_ArrayIsListJitHelper__islist in hash_hmac_algos fixture (array_is_list bridge).

Root cause

build/helper-runtime-cache/units/ext_standard_ArrayIsListJitHelper_php/unit.o was 0 bytes while manifest/bc looked fresh. Build cache outranks prelinked in helperIndex(), so tryProvide bound an extern declaration but linked an empty object.

Test plan

./script/aot-smoke.sh
# aot-smoke: 8 passed, 0 failed

./script/phpunit.sh --filter 'HelperRuntimeCacheFingerprintTest::testUnitObjectIsLinkableRejectsZeroByteObject|HelperRuntimeCacheFingerprintTest::testHelperIndexFallsBackToPrelinkedWhenBuildCacheObjectEmpty|AotTest::testCases with data set "hash_hmac_algos"'
# OK (3 tests, 10 assertions)

Relates #6229 / #15889

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