YJIT: No need to fill to get UDF on ARM64

On ARM64, all zeros is already undefined, so we don't need to do extra
work to fill new memory with undefined instructions.

yjit/src/virtualmem.rs

@@ -141,10 +141,16 @@ impl<A: Allocator> VirtualMemory<A> {
                     if !alloc.mark_writable(mapped_region_end.cast(), alloc_size_u32) {
                         return Err(FailedPageMapping);
                     }
-                    // Fill new memory with PUSH DS (0x1E) so that executing uninitialized memory
-                    // will fault with #UD in 64-bit mode. On Linux it becomes SIGILL and use the
-                    // usual Ruby crash reporter.
-                    std::slice::from_raw_parts_mut(mapped_region_end, alloc_size).fill(0x1E);
+                    if cfg!(target_arch = "x86_64") {
+                        // Fill new memory with PUSH DS (0x1E) so that executing uninitialized memory
+                        // will fault with #UD in 64-bit mode. On Linux it becomes SIGILL and use the
+                        // usual Ruby crash reporter.
+                        std::slice::from_raw_parts_mut(mapped_region_end, alloc_size).fill(0x1E);
+                    } else if cfg!(target_arch = "aarch64") {
+                        // In aarch64, all zeros encodes UDF, so it's already what we want.
+                    } else {
+                        unreachable!("unknown arch");
+                    }
                 }
                 self.mapped_region_bytes = self.mapped_region_bytes + alloc_size;