php-in-php: strnatcmp/strnatcasecmp LLVM standalone bodies (#5517)

[PurHur]

Summary

• Close out php-in-php: strnatcmp()/strnatcasecmp() — delete phpc_strnatcmp.c + phpc_strnatcasecmp.c (~155 lines) #5517: phpc_strnatcmp.c / phpc_strnatcasecmp.c were already deleted from lib/AOT/runtime/; this PR emits LLVM bodies for AOT standalone load type (matching LastErrorRuntime pattern) so linked binaries do not rely on external C symbols.
• Add JIT compliance .phpt, AOT fixture, and standalone unit test.

C runtime shrink

• ~155 lines already removed (prior merges): lib/AOT/runtime/phpc_strnatcmp.c, phpc_strnatcasecmp.c — not in Linker.php.
• This PR removes the standalone early-return that left strnatcmp/strnatcasecmp as declaration-only stubs.

php-src reference

• ext/standard/string.c — PHP_FUNCTION(strnatcmp), PHP_FUNCTION(strnatcasecmp)
• PHP implementation: ext/standard/VmString.php, lib/JIT/Builtin/StringNaturalCompareJit.php

Verification

# Issue repro (Zend parity)
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php -r "var_export(strnatcmp(\"2\", \"10\"));"'
# => -1

php bin/vm.php -r "var_export(strnatcmp('2', '10'));"
php bin/jit.php -r "var_export(strnatcmp('2', '10'));"

# Confirm C absent
! ls lib/AOT/runtime/phpc_strnat*.c 2>/dev/null
! grep -q strnatcmp lib/AOT/Linker.php

# Targeted PHPUnit
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/compliance/StrnatcmpJITTest.php'

PHPUnit 9.6.21 … OK (1 test, 1 assertion)

Closes #5517

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