YJIT: Fix IseqPayload::pages memory bloat

HashSet::clear() doesn't deallocate the backing buffer and shrink the
capacity. Replace with a 0-capcity set instead so we reclaim some memory
each code GC.

yjit/src/asm/mod.rs

@@ -574,7 +574,7 @@ impl CodeBlock {
         });
         // Avoid accumulating freed pages for future code GC
         for_each_off_stack_iseq_payload(|iseq_payload: &mut IseqPayload| {
-            iseq_payload.pages.clear();
+            iseq_payload.pages = std::collections::HashSet::default();
         });
         // Outlined code generated by CodegenGlobals::init() should also be kept.
         for page in CodegenGlobals::get_ocb_pages() {