Consistify polling frequency of llama-bench with args

llama-bench differs from llama-server in 2 ways:
1. It attaches a persistent threadpool across cgraph invocations
2. It has a polling scheme of 50 (as opposed to 0 of args)

Together, this causes a significant perf regression in the CUDA backend
on Windows when the CPU backend is built with OpenMP disabled.
Why is the CPU backend affecting the CUDA backend?
Typically, we have a LUT on the CPU backend where we map Token_ID
-> Embedding to save on VRAM budget.

tools/llama-bench/llama-bench.cpp

@@ -380,7 +380,7 @@ static const cmd_params cmd_params_defaults = {
     /* n_threads            */ { common_cpu_get_num_math() },
     /* cpu_mask             */ { "0x0" },
     /* cpu_strict           */ { false },
-    /* poll                 */ { 50 },
+    /* poll                 */ { 0 },
     /* n_gpu_layers         */ { -1 },
     /* n_cpu_moe            */ { 0 },
     /* split_mode           */ { LLAMA_SPLIT_MODE_LAYER },