Force use a signed char (On ARM char is unsigned by default)

[phprus]

Environment:

docker-desktop:/ # uname -a
Linux docker-desktop 5.15.49-linuxkit #1 SMP PREEMPT Tue Sep 13 07:51:32 UTC 2022 aarch64 aarch64 aarch64 GNU/Linux

Error:

[ RUN      ] chrono_test.cpp20_duration_subsecond_support
/fmt/fmt-master/test/chrono-test.cc:823: Failure
Expected equality of these values:
  fmt::format("{:%S}", std::chrono::duration<char, std::ratio<1, 100>>(0x80))
    Which is: "01.28"
  "-01.28"
[  FAILED  ] chrono_test.cpp20_duration_subsecond_support (0 ms)

On ARM gcc char is unsigned:
https://developer.arm.com/documentation/den0013/d/Porting/Miscellaneous-C-porting-issues/unsigned-char-and-signed-char
https://trofi.github.io/posts/203-signed-char-or-unsigned-char.html

[vitaut]

Thanks