fix #16033 nim js --gc:arc works and ignores --gc:arc (#16036)

(cherry picked from commit 557dcfd87d)

compiler/commands.nim

@@ -485,7 +485,10 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
   of "project":
     processOnOffSwitchG(conf, {optWholeProject, optGenIndex}, arg, pass, info)
   of "gc":
-    if conf.backend == backendJs:
+    if conf.backend == backendJs or conf.command == "js":
+      # for: bug #16033
+      # This might still be imperfect, in rarse corner cases
+      # (where command is reset in nimscript, maybe).
       return
     expectArg(conf, switch, arg, pass, info)
     if pass in {passCmd2, passPP}:

tests/arc/t16033.nim

@@ -0,0 +1,10 @@
+discard """
+  targets: "c js"
+  matrix: "--gc:arc"
+"""
+
+# bug #16033
+when defined js:
+  doAssert not compileOption("gc", "arc")
+else:
+  doAssert compileOption("gc", "arc")