build: Use signed char in RISC-V 64bit

[cosmo0920]

Same as other platforms, we have to use signed char even if RISC-V 64bit(riscv64).
For testing on RISCV64, we can also use uraimo/run-on-arch-action action to set up with riscv64 emulator with qemu.

In my visionfive RISC-V 64bit board, I succeeded to pass signedness sensitive tests which are flb-it-pack and flb-it-utils.
This is also causing problems on AArch64 Linux.
And there is also occurring on RISCV64 Linux.

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