Fix non-ASCII path handling in common argument parser on Windows

[kuguma]

Summary

Fixed the behavior of the llama.cpp general argument parser when given non-ASCII paths.

On Windows, argv strings are received in the system’s ANSI code page (ACP), so conversion to UTF-8 is required.

Comparison of behavior

Windows 11 Pro 24H2

PS > Get-WinSystemLocale
LCID             Name             DisplayName
----             ----             -----------
1041             ja-JP            Japanese (Japan)
PS > [Console]::InputEncoding.EncodingName
Japanese (Shift-JIS)
PS > [Console]::OutputEncoding.EncodingName
Unicode (UTF-8)

b6756

PS > llama-cli.exe -m "..\モデル\gemma-3-4b-it-Q4_K_M.gguf"
build: 6756 (bc07349a7) with MSVC 19.44.35214.0 for x64
main: llama backend init
main: load the model and apply lora adapter, if any
gguf_init_from_file: failed to open GGUF file '..\���f��\gemma-3-4b-it-Q4_K_M.gguf'
llama_model_load: error loading model: llama_model_loader: failed to load model from ..\���f��\gemma-3-4b-it-Q4_K_M.gguf
llama_model_load_from_file_impl: failed to load model
common_init_from_params: failed to load model '..\���f��\gemma-3-4b-it-Q4_K_M.gguf', try reducing --n-gpu-layers if you're running out of VRAM
main: error: unable to load model

This PR

PS > llama-cli.exe -m "..\モデル\gemma-3-4b-it-Q4_K_M.gguf"
build: 6758 (7ae5e44d4) with MSVC 19.44.35214.0 for x64
main: llama backend init
main: load the model and apply lora adapter, if any
llama_model_loader: loaded meta data with 40 key-value pairs and 444 tensors from ..\モデル\gemma-3-4b-it-Q4_K_M.gguf (version GGUF V3 (latest))
llama_model_loader: Dumping metadata keys/values. Note: KV overrides do not apply in this output.
~~~
> 

Note

This PR is related to

• mtmd : Fix/Add non-ASCII file path support on Windows #16609.

Reproducing the issue with non-ASCII .mmproj paths in existing CLI tools requires first applying the changes from this PR. (This doesn’t apply when the tools are called internally.)