Update system desc with CPU device information

[nsmithtt]

System descriptor is tt-mlir's means for capturing all relevant information about the target system needed for compile. This is the perfect place to store CPU information for executing operations on the host.

Update table SystemDesc with CPU device information, perhaps something like:

enum CPURole: uint16 {
  Host = 0,
  Device = 1,
}

table CPUDesc {
  role: CPURole;
  target_triple: string;
}

table SystemDesc {    
  ...
  cpu_descs: [CPUDesc];
}

Where target triple can be scraped from the current machine via:

clang -dumpmachine
# OR
gcc -dumpmachine

We should collect this information during ttrt's setup.py script and pass this host's target triple information via preprocessor macro -DHOST_TARGET_TRIPLE=$($CC -dumpmachine).