zend: fix missing return in global register detection (phpGH-22206) - #22319

Hi,
this pull request add a missing return statement to zend global registers detection, fixing a build failure with -Werror=return-type with GCC 16.

When building with -Werror=return-type, the global registers detection ends with an error, even though global registers are supported. The preserve_none attribute is newly supported, which along with "unsupported" global registers leads to ZEND_VM_KIND being set to ZEND_VM_KIND_TAILCALL, which later produces the error described in the linked issue. This pull request fixes that.