There are 0.9V microcontrollers, for example from Seiko/Epson, but
they're ROM based. I'm not aware of any Flash (whether on chip with a
microcontroller or separate) that goes down in Voltage that far. Your
best bet will be to use a DC/DC converter to step up the voltage.
I think the former actually incorporate an on-chip charge pump to
double the cell voltage with the help of a couple of external caps.
Some Japanese mask micros have a mask option to select straight
through or boosted input PS voltage.
For smaller quantity or more demanding applications, an external
doubler, an extra cell, or a higher voltage cell may be the best
solution.
Best regards,
Spehro Pefhany