x86: gen_idt.py: Use enumerate() to fix pylint warning

Use enumerate() to fix this pylint warning:

    C0200: Consider using enumerate instead of iterating with range and
    len (consider-using-enumerate)

enumerate() is handy when the loop body needs both the element and its
index. It returns (index, element) tuples.

Also use a tuple unpacking to extract 'handler' from the elements in
'vector'.

Piggyback a slightly simpler way to build a list of num_chars 0s.

Getting rid of warnings for a CI check.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>