Fixed codegen for constant cstring with --gc:arc (#13326)

* Fixed codegen for constant cstring with --gc:arc, fixes  #13321

* Added test for #13321

tests/arc/tcstring.nim

@@ -0,0 +1,16 @@
+discard """
+  cmd: "nim c --gc:arc -r $file"
+  nimout: '''hello
+h
+o
+'''
+"""
+
+# Issue #13321: [codegen] --gc:arc does not properly emit cstring, results in SIGSEGV
+
+let a = "hello".cstring
+echo a
+echo a[0]
+echo a[4]
+doAssert a[a.len] == '\0'
+