x64: Port func_addr and symbol_value to ISLE (#4485)

https://github.com/bytecodealliance/wasmtime/pull/4485

cranelift/codegen/src/isa/x64/inst.isle

@@ -1488,6 +1488,13 @@
 
 ;;;; Helpers for Emitting Loads ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
+;; Helper for constructing a LoadExtName instruction.
+(decl load_ext_name (ExternalName i64) Reg)
+(rule (load_ext_name extname offset)
+      (let ((dst WritableGpr (temp_writable_gpr))
+            (_ Unit (emit (MInst.LoadExtName dst extname offset))))
+        dst))
+
 ;; Load a value into a register.
 (decl x64_load (Type SyntheticAmode ExtKind) Reg)