Update error message to use "ld.lld" instead of "lld -flavor gnu".

[ruiu]

Summary

Dispatching based on argv[0] seems to be more convenient for users
than dispatching based on -flavor option. Currently, when a user
invoke LLD as "lld", we recommend them pass -flavor option. This
patch changes the message so that we recommend use ld.lld, ld or
lld-link instead.

lld/trunk/tools/lld/lld.cpp

int main(int Argc, const char **Argv) {
   switch (parseFlavor(Args)) {
   case Gnu:
     return !elf::link(Args);
   case WinLink:
     return !coff::link(Args);
   case Darwin:
     return !mach_o::link(Args);
   default:
-    die("-flavor option is missing. Available flavors are "
-        "gnu, darwin or link.");
+    die("lld is a generic driver.\n"
+        "Invoke ld.lld (Unix), ld (Mac) or lld-link (Windows) instead.");
   }
 }