YJIT: Fix jmp_ptr_bytes on x86_64 (#9016)

yjit/src/backend/x86_64/mod.rs

@@ -32,7 +32,7 @@ pub const _C_RET_OPND: Opnd = Opnd::Reg(RAX_REG);
 
 impl CodeBlock {
     // The number of bytes that are generated by jmp_ptr
-    pub fn jmp_ptr_bytes(&self) -> usize { 6 }
+    pub fn jmp_ptr_bytes(&self) -> usize { 5 }
 }
 
 /// Map Opnd to X86Opnd