<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def get_arch_string():
    r'''
    Check local or remote system arch, to produce TaskCluster proper link.
    '''
    rc, stdout, stderr = exec_command('uname -sm')
    if rc > 0:
        raise AssertionError('Error checking OS')

    stdout = stdout.lower().strip()
    if not 'linux' in stdout:
        raise AssertionError('Unsupported OS')

    if 'armv7l' in stdout:
        return 'arm'

    if 'x86_64' in stdout:
        nv_rc, nv_stdout, nv_stderr = exec_command('nvidia-smi')
        nv_stdout = nv_stdout.lower().strip()
        if 'NVIDIA-SMI' in nv_stdout:
            return 'gpu'
        else:
            return 'cpu'

    raise AssertionError('Unsupported arch:', stdout)