fix nim-lang#12735 osx: dsymutil needs to be called for debug builds

compiler/extccomp.nim

@@ -838,6 +838,14 @@ proc execLinkCmd(conf: ConfigRef; linkCmd: string) =
     execExternalProgram(conf, linkCmd,
       if optListCmd in conf.globalOptions or conf.verbosity > 1: hintExecuting else: hintLinking)
 
+proc maybeRunDsymutil(conf: ConfigRef; exe: AbsoluteFile) =
+  when not defined(osx): return
+  if optCDebug notin conf.globalOptions: return
+  # if needed, add an option to skip or override location
+  let cmd = "dsymutil " & $(exe).quoteShell
+  tryExceptOSErrorMessage(conf, "invocation of dsymutil failed."):
+    execExternalProgram(conf, cmd, hintExecuting)
+
 proc execCmdsInParallel(conf: ConfigRef; cmds: seq[string]; prettyCb: proc (idx: int)) =
   let runCb = proc (idx: int, p: Process) =
     let exitCode = p.peekExitCode
@@ -979,6 +987,7 @@ proc callCCompiler*(conf: ConfigRef) =
           linkViaResponseFile(conf, linkCmd)
         else:
           execLinkCmd(conf, linkCmd)
+        maybeRunDsymutil(conf, mainOutput)
   else:
     linkCmd = ""
   if optGenScript in conf.globalOptions: