gc: fix missing pthread registration causing unknown thread error

- Use `GC_pthread_create` instead of `pthread_create` when GC is
  enabled.

ssa/goroutine.go

@@ -51,7 +51,7 @@ func (p Program) tyPthreadCreate() *types.Signature {
 }
 
 func (b Builder) pthreadCreate(pp, attr, routine, arg Expr) Expr {
-	fn := b.Pkg.cFunc("pthread_create", b.Prog.tyPthreadCreate())
+	fn := b.Pkg.cFunc("llgoPthreadCreate", b.Prog.tyPthreadCreate())
 	return b.Call(fn, pp, attr, routine, arg)
 }