aco: Fixes compiling error about char8_t with c++20

aco: Fixes compiling error about char8_t with c++20

The error is:
../mesa/src/amd/compiler/aco_register_allocation.cpp:382:7: error: no matching function for call to 'printf'
      printf(u8"☐");

Fixes: 209a89e51d1 ("aco: Convert to use u8 literal for Unicode character to fixes msvc warning")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7318

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Marcus Seyfarth <m.seyfarth@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18796>