Generate dynlib strings as a single rope (#8247)

In order not to trip the optimization in genInfixCall we have to do so.
The same trick is also used in setExternName.

Fixes #8241

tests/cpp/t8241.nim

@@ -0,0 +1,7 @@
+discard """
+  targets: "cpp"
+  action: "compile"
+"""
+
+proc foo(): cstring {.importcpp: "", dynlib: "".}
+echo foo()