Port FuncAddr & SymbolValue to ISLE (AArch64) (#4748)

Ported the existing implementations of the following opcodes for AArch64
to ISLE:
- `FuncAddr`
- `SymbolValue`

Copyright (c) 2022 Arm Limited

cranelift/codegen/src/isa/aarch64/lower.isle

@@ -1832,6 +1832,16 @@
 (rule (lower (debugtrap))
       (side_effect (brk)))
 
+;;;; Rules for `func_addr` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (func_addr (func_ref_data _ extname _)))
+      (load_ext_name (box_external_name extname) 0))
+
+;;;; Rules for `symbol_value` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (symbol_value (symbol_value_data extname _ offset)))
+      (load_ext_name (box_external_name extname) offset))
+
 ;;; Rules for `get_{frame,stack}_pointer` and `get_return_address` ;;;;;;;;;;;;;
 
 (rule (lower (get_frame_pointer))