DBI: machine type - should differentiate UNKNOWN (0x0) from indeterminate types

[wesinator]

for PE machine type values - UNKNOWN is a specific label associated with value 0x0

There can be other machine type values that don't match the enum.
Currently these will also return UNKNOWN label:

pdb/src/dbi.rs

Line 371 in 65c5b6d

_ => Self::Unknown,

This should probably be changed to be granular about non-standard machine types.