Fix bad architecture detection on Raspbian Wheezy

On Raspbian Wheezy, there is no "vendor_id" in the /proc/cpuinfo
so the regex on line 94:

    regexp = re.compile(r"^vendor_id\s+:\s*(?P<vendorid>\S+)\s*$", re.M)

will fail, and there will be no groupdict() returned from the
following line:

    result = regexp.search(txt).groupdict()

So I wrapped this in a "try/except AttributeError" and so that
this could proceed to the ARM detection.