php-in-php: drop JitEnv putenv thin libc fork — always GetenvJitHelper (#21023)

[PurHur]

Summary

• Delete isThinStandaloneAotMain libc-only fork from JitEnv::putenv / putenvFromCStringLiteral — thin + embed always NestedJIT GetenvJitHelper::putenv + libc setenv mirror (peer getenv php-in-php: drop isThinStandaloneAotMain libc fork from StringGetenv — always JitVmHelperLink + GetenvJitHelper (re-#20156) #20644 / rename php-in-php: drop isThinStandaloneAotMain libc fork from StringRename — always JitVmHelperLink + RenameJitHelper (re-#20028) #20603).
• Literal putenv returns bare int1 so Internal::call type inference works (putenv_smoke no longer dies on int8).
• Shrink test guards: no thin gate remains in JitEnv.php.

C runtime: no new runtime/*.c; net shrink is the thin LLVM early-return (~15 LOC of forked emit path).

Closes #21023

Test plan

• vendor/bin/phpunit test/unit/PutenvRuntimeShrinkTest.php → OK (2 tests, 11 assertions)
• ./phpc build -o /tmp/putenv_smoke test/bootstrap-aot/putenv_smoke.php && /tmp/putenv_smoke → putenv ok
• make bootstrap-selfhost-vm-driver-execute-probe → OK

Verification transcript

OK (2 tests, 11 assertions)
putenv ok
smoke_ok
bootstrap-selfhost-vm-driver-execute-probe: OK

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