Suppress unused label warnings in SWITCH VM

While we avoid emitting labels for handlers that are not referenced
from anywhere else, we do not perform a fine-grained analysis on
used specializations, so some of the specialization labels may not
be used. Use ATTRIBUTE_UNUSED_LABEL to suppress the warning. Drop
"cold" from the definition of this attribute, as it is completely
unrelated.

Zend/zend_portability.h

@@ -245,7 +245,7 @@ char *alloca();
 #endif
 
 #if defined(__GNUC__) && ZEND_GCC_VERSION >= 5000
-# define ZEND_ATTRIBUTE_UNUSED_LABEL __attribute__((cold, unused));
+# define ZEND_ATTRIBUTE_UNUSED_LABEL __attribute__((unused));
 # define ZEND_ATTRIBUTE_COLD_LABEL __attribute__((cold));
 # define ZEND_ATTRIBUTE_HOT_LABEL __attribute__((hot));
 #else

Zend/zend_vm_gen.php

@@ -1123,7 +1123,7 @@ function gen_handler($f, $spec, $kind, $name, $op1, $op2, $use, $code, $lineno,
             }
             if ($use) {
                 // This handler is used by other handlers. We will add label to call it.
-                out($f," {$spec_name}_LABEL:\n");
+                out($f," {$spec_name}_LABEL: ZEND_ATTRIBUTE_UNUSED_LABEL\n");
             } else {
                 out($f,"\n");
             }