don't use {.rtl.} for generics, otherwise -d:useNimRtl gives ambiguous identifier nimrtl error (#18406)

lib/pure/strutils.nim

@@ -1760,7 +1760,7 @@ func join*(a: openArray[string], sep: string = ""): string {.rtl,
   else:
     result = ""
 
-func join*[T: not string](a: openArray[T], sep: string = ""): string {.rtl.} =
+func join*[T: not string](a: openArray[T], sep: string = ""): string =
   ## Converts all elements in the container `a` to strings using `$`,
   ## and concatenates them with `sep`.
   runnableExamples:

lib/system/inclrtl.nim

@@ -30,12 +30,13 @@ when defined(createNimRtl):
   {.pragma: inl.}
   {.pragma: compilerRtl, compilerproc, exportc: "nimrtl_$1", dynlib.}
 elif defined(useNimRtl):
-  when defined(windows):
+  #[
-    const nimrtl* = "nimrtl.dll"
+  `{.rtl.}` should only be used for non-generic procs.
-  elif defined(macosx):
+  ]#
-    const nimrtl* = "libnimrtl.dylib"
+  const nimrtl* =
-  else:
+    when defined(windows): "nimrtl.dll"
-    const nimrtl* = "libnimrtl.so"
+    elif defined(macosx): "libnimrtl.dylib"
+    else: "libnimrtl.so"
   {.pragma: rtl, importc: "nimrtl_$1", dynlib: nimrtl, gcsafe.}
   {.pragma: inl.}
   {.pragma: compilerRtl, compilerproc, importc: "nimrtl_$1", dynlib: nimrtl.}

tests/dll/client.nim

@@ -1,5 +1,4 @@
 discard """
-  output: "Done"
   cmd: "nim $target --debuginfo --hints:on --define:useNimRtl $options $file"
 """
 
@@ -37,5 +36,8 @@ proc eval(n: PNode): int =
 for i in 0..100_000:
   discard eval(buildTree(2))
 
-echo "Done"
+# bug https://forum.nim-lang.org/t/8176; Error: ambiguous identifier: 'nimrtl'
-
+import std/strutils
+doAssert join(@[1, 2]) == "12"
+doAssert join(@[1.5, 2.5]) == "1.52.5"
+doAssert join(@["a", "bc"]) == "abc"