SPIRVIntrinsics: Unknown type Int8 on aarch64-linux

[maleadt]

Apparently both char and cuchar are unsigned on this platform, so we need to fix how Int8 intrinsics are mangled (or use different intrinsics for Int8). Probably by checking how an int8_t call (which I presume is still signed on linux-aarch64) is compiled by OpenCL compilers.

[giordano]

Apparently what makes things even more complicated is that this is really compiler-dependent (as most types in C), not just platform: https://stackoverflow.com/a/20576301 suggests that this can be changed with an option in XCode for macOS platforms, and there's an llvm option to choose the signedness of char.