Stdlib: withhold gmp on reference profile without host php-gmp (#22860)

[PurHur]

Summary

• Gate ext/gmp advertisement on host extension_loaded('gmp') or PHP_COMPILER_PROFILE=8.4 (CompilerVersion::supportsGmp()), matching Zend builds without php-gmp.
• Withhold gmp_* / GMP class / module registration on the reference harness; skip functional gmp PHPTs until advertised (phantom kept).
• php-src: ext/gmp/gmp.c; PHP in ext/gmp + introspection/module registry.

Closes #22860

Test plan

• php bin/vm.php test/repro/extension_loaded_gmp_phantom.php → all no (matches Zend)
• PHP_COMPILER_PROFILE=8.4 php bin/vm.php … → all yes
• vendor/bin/phpunit --filter GmpExtensionPolicyTest → OK
• vendor/bin/phpunit --filter extension_loaded_gmp_phantom → OK (2)
• php script/bootstrap-inventory.php --check → OK 6519/6519

Verification transcript

=== ZEND ===
ext=no gmp_add=no GMP=no
=== VM default ===
ext=no gmp_add=no GMP=no
=== VM PROFILE=8.4 ===
ext=yes gmp_add=yes GMP=yes
=== UNIT ===
OK (1 test, 4 assertions)
=== COMPLIANCE phantom ===
OK (2 tests, 2 assertions)
=== inventory ===
OK 6519/6519

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