Add a new kind of GlobalVar for symbolic addresses.

These addresses will allow referencing C/C++/Rust-style global variables
by name directly.

cranelift/filetests/parser/memory.cton

@@ -36,6 +36,20 @@ ebb0:
     return v1
 }
 
+function %sym() -> i32 {
+    gv0 = globalsym %something
+    ; check: $gv0 = globalsym %something
+    gv1 = globalsym #d0bad180d0b5d182d0bed0bd
+    ; check: $gv1 = globalsym #d0bad180d0b5d182d0bed0bd
+ebb0:
+    v0 = global_addr.i32 gv0
+    ; check: $v0 = global_addr.i32 $gv0
+    v1 = global_addr.i32 gv1
+    ; check: $v1 = global_addr.i32 $gv1
+    v2 = bxor v0, v1
+    return v2
+}
+
 ; Declare static heaps.
 function %sheap(i32) -> i64 {
     heap1 = static reserved_reg, min 0x1_0000, bound 0x1_0000_0000, guard 0x8000_0000