php-in-php: lower locale_get_default() on AOT/JIT (#27369)

[PurHur]

Summary

• Remove the locale_get_default() JIT/AOT defer refusal (Stdlib: locale_get_default()/locale_set_default() and Locale class missing (ext/intl/php_intl.c) #9576) by routing through JitVmHelperLink → new slim LocaleDefaultJitHelper
• LocaleDefaultJitHelper mirrors VmLocale::getDefault() semantics (ini + LC_* env) without NestedJIT-compiling all of VmLocale (ICU FFI static init SEGV'd under user-script AOT)
• VM execute() path unchanged; JITTest skip for locale_get_default removed

php-src reference

• ext/intl/locale/locale_methods.c — locale_get_default

C runtime shrink

No new C. Behavior lives in ext/intl/LocaleDefaultJitHelper.php + lib/JIT/Builtin/LocaleParser.php.

Verification

# Issue repro — was compile exit 2 "deferred; use VM"
cat >/tmp/locale_get_default.php <<'P'
<?php
echo locale_get_default(), PHP_EOL;
P
php /tmp/locale_get_default.php          # en_US_POSIX
php bin/vm.php /tmp/locale_get_default.php  # en_US_POSIX
php bin/compile.php -o /tmp/locale_get_default.bin /tmp/locale_get_default.php && /tmp/locale_get_default.bin  # en_US_POSIX

vendor/bin/phpunit test/unit/LocaleGetDefaultRuntimeShrinkTest.php test/unit/LocaleParserRuntimeShrinkTest.php
make bootstrap-selfhost-vm-driver-execute-probe  # OK
php script/check-selfhost-spine-coverage-sync.php  # OK 7007/7007

Not run: full ci-fast.sh (in flight), compliance locale_get_default.phpt shard, locale_set_default (still deferred).

Closes #27369

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