undefine C symbols in JS backend [backport:1.6] (#19437)

fix #19330; fix #19059

compiler/cmdlinehelper.nim

@@ -64,7 +64,8 @@ proc loadConfigsAndProcessCmdLine*(self: NimProg, cache: IdentCache; conf: Confi
       if conf.cmd == cmdNimscript: return false
   # now process command line arguments again, because some options in the
   # command line can overwrite the config file's settings
-  extccomp.initVars(conf)
+  if conf.backend != backendJs: # bug #19059
+    extccomp.initVars(conf)
   self.processCmdLine(passCmd2, "", conf)
   if conf.cmd == cmdNone:
     rawMessage(conf, errGenerated, "command missing")

compiler/commands.nim

@@ -899,8 +899,9 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     splitSwitch(conf, arg, key, val, pass, info)
     os.putEnv(key, val)
   of "cc":
-    expectArg(conf, switch, arg, pass, info)
-    setCC(conf, arg, info)
+    if conf.backend != backendJs: # bug #19330
+      expectArg(conf, switch, arg, pass, info)
+      setCC(conf, arg, info)
   of "track":
     expectArg(conf, switch, arg, pass, info)
     track(conf, arg, info)

tests/js/tcsymbol.nim

@@ -0,0 +1,6 @@
+discard """
+  matrix: "--cc:gcc; --cc:tcc"
+"""
+
+doAssert not defined(gcc)
+doAssert not defined(tcc)