Support Atmega8 (and similar that define PE instead of UPE) again

These chips were previously supported, but since parity error checking
was added, this support has broken. Most chips define UPE0 (etc.) for
the parity error bit. Some chips don't have numbered UARTS so only
define UPE and even fewer define PE instead of UPE. This adds support
for those chips again.

Closes: #2137