--threads is a flag, not a number

We would previously accept `--threads=4`, but this option just turns on
threading and does not specify a number of threads.

I ran into this by accident because I was passing `--threads=<n>` but
the number didn't seem to affect anything.

llvm-svn: 270963

Author: chisophugis

lld/ELF/Options.td

@@ -150,7 +150,7 @@ def strip_debug : Flag<["--"], "strip-debug">,
 def sysroot : Joined<["--"], "sysroot=">,
   HelpText<"Set the system root">;
 
-def threads : Joined<["--"], "threads">;
+def threads : Flag<["--"], "threads">;
 
 def trace: Flag<["--"], "trace">,
   HelpText<"Print the names of the input files">;