Toolchain: Don't pass -lpthread when pthreads are requested

This is essentially a no-op since we have our replacement linker script,
but if we can skip over that entirely, why not?

Toolchain/Patches/llvm/0003-Driver-Add-support-for-SerenityOS.patch

@@ -144,7 +144,7 @@ new file mode 100644
 index 000000000..955422438
 --- /dev/null
 +++ b/clang/lib/Driver/ToolChains/Serenity.cpp
-@@ -0,0 +1,340 @@
+@@ -0,0 +1,337 @@
 +//===---- Serenity.cpp - SerenityOS ToolChain Implementation ----*- C++ -*-===//
 +//
 +// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
@@ -306,9 +306,6 @@ index 000000000..955422438
 +  }
 +
 +  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
-+    if (Args.hasArg(options::OPT_pthread, options::OPT_pthreads))
-+      CmdArgs.push_back("-lpthread");
-+
 +    if (!Args.hasArg(options::OPT_nolibc))
 +      CmdArgs.push_back("-lc");
 +  }