Signed/unsigned char on different platforms

I'm not sure how much difference it makes in practice, but the `char` type in C/C++ isn't guaranteed to be signed or unsigned.

By default, x86/M1 seem to treat char as signed, but ppc64le treats it as unsigned. This can also be modified by compiler options (e.g., https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc/C-Dialect-Options.html#index-funsigned-char).

https://github.com/inducer/compyte/blob/d4549d4c711513e2cc098d3f5d4e918eac53ee7a/dtypes.py#L122

OpenCL seems to define it as signed: https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_C.html#built-in-scalar-data-types