Fix AOT array_first/array_last wrong output under PROFILE=8.5 (#27596)

[PurHur]

Summary

• Add HashTableElemLlvm pure-LLVM path for array_first() / array_last() under thin standalone AOT (peer HashTableShiftLlvm / REGRESSION: e30_array_lit_dim_assign_shift no longer compiles — HashTable::shiftFirst() unresolved #24025).
• Route ArrayElemRuntime::first/last through LLVM when isThinStandaloneAotMain(); JIT/MCJIT keep ArrayElemJitHelper php-in-PHP bridge.
• Extend JitVmHelperLink::ensureBridge / ensureCompiled with optional skipHelperRuntimeCache for future thin-AOT helper bundles.

Problem

NestedJIT ArrayElemJitHelper Variable returns were wrong (NULL / Object) or segfaulted under phpc build while VM/JIT matched Zend (#27596).

Verification

export PHP_COMPILER_PROFILE=8.5
# Issue repro (echo — json_encode still aborts separately under thin AOT #23540)
cat > /tmp/issue27596.php <<'PHP'
<?php
echo array_first([10, 20]), ",", array_last([10, 20]), ",", var_export(array_first([]), true), "\n";
PHP
php bin/vm.php /tmp/issue27596.php          # 10,20,NULL
php bin/compile.php -o /tmp/issue27596 /tmp/issue27596.php && /tmp/issue27596  # 10,20,NULL exit=0

php vendor/bin/phpunit --filter array_first_last test/aot/AotTest.php  # OK (4 tests)
php vendor/bin/phpunit test/unit/ArrayElemRuntimeShrinkTest.php       # OK (5 tests)

Not covered

• json_encode([...]) in the issue repro still aborts under thin AOT (AOT: var_dump() aborts (SIGABRT) with no diagnostic — even var_dump(7) #23540) — orthogonal to array_first/last values.
• Full north-star5-verify --strict / bootstrap blob refresh.
• Helper-runtime prelink refresh for ArrayElemJitHelper (thin AOT no longer uses cached unit for these builtins).

Closes #27596

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