php-in-php: drop always-on LibcExtern strlen after NestedJIT module-local (#32068)

[PurHur]

Summary

• Drop the always-on LibcExtern strlen(3) row. User-script strlen() already lives in PHP (ext/types/strlen.php + JitStrlen / VmString::byteLength).
• NestedJIT C-string length leaves call LibcExtern::ensureStrlenDecl() before lookupFunction('strlen'). EMBED MCJIT still gets implementStrlenBody after the module-local decl.
• Drop the Module::jitInit always-on strlen fallback (peer php-in-php: drop always-on LibcExtern strtod after NestedJIT module-local (#31988 shape) #31997 strtod).

Closes #32068

php-src: ext/standard/string.c PHP_FUNCTION(strlen) / Zend Z_STRLEN — not libc strlen on user values.

C runtime: no runtime/*.c added. Always-on libc strlen row removed from lib/JIT/LibcExtern.php ('strlen' => gone); thin ensureStrlenDecl() trampoline only.

Test plan

• ./script/phpunit.sh --filter 'LibcExternDeadDeclsRuntimeShrinkTest|StrlenRuntimeShrinkTest|LibcExternMathRuntimeShrinkTest'
• ./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/nested_trim_strlen.php' → 5

PHPUnit 9.6.21 by Sebastian Bergmann and contributors.

.................................                                 33 / 33 (100%)

Time: 00:00.221, Memory: 205.73 MB

OK (33 tests, 583 assertions)

Did not run make test-harness / ci-local.sh / bootstrap link (not compile/link/self-host).

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