php-in-php: drop always-on LibcExtern malloc/realloc/free (#32273)

[PurHur]

Summary

• Drop always-on libc malloc(3) / realloc(3) / free(3) from LibcExtern and declare them module-locally via ensureMallocFamily() (canonical i8* / size_t).
• NestedJIT leaves that used void* or i64 size no longer mint malloc.1 (the Fix: AOT link fails for EVERY binary — duplicate memset declaration #31894 / Release v1.1.0: fix cold-build-check compile_failed on hello world (LLVM Module.php line 180; re-#24302) #32122 signature-clash class). User-script allocation stays MemoryManager __mm__malloc / __mm__realloc / __mm__free.
• php-src: Zend/zend_alloc.c / Zend/zend_alloc.h (Zend heap); libc malloc remains the LLVM C-buffer floor.

Closes #32273

Test plan

• ./script/phpunit.sh --filter 'MallocRuntimeShrinkTest|LibcExternDeadDeclsRuntimeShrinkTest'
• Full ./script/aot-smoke.sh (8/8) — not completed this run: first AOT compile was still in NestedJIT helper emission (ReadfileJitHelper::readfile) at 90s after the LibcExtern.php fingerprint change; did not wait for a whole-corpus helper rebuild (20 min cap). Source shrink gates are the issue repro.

./script/phpunit.sh --filter 'MallocRuntimeShrinkTest|LibcExternDeadDeclsRuntimeShrinkTest'
PHPUnit 9.6.21 by Sebastian Bergmann and contributors.

................................                                  32 / 32 (100%)

Time: 00:00.413, Memory: 213.77 MB

OK (32 tests, 651 assertions)

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