Bug#877754: NaN -> datetime = 0 (not NaT) on arm*

[Rebecca N. Palmer]

Control: retitle -1 NaN -> datetime = 0 (not NaT) on arm*
Control: tag -1 - fixed-upstream
Control: reassign -1 python3-numpy
Control: affects -1 python3-pandas
Control: forwarded -1 https://github.com/numpy/numpy/issues/8325

The underlying issue is that datetime/timedelta are internally ints, and 
use the C float -> int conversion.  This is undefined when the input is 
NaN, so is allowed to be whatever is most efficient on that hardware: 
typically -2**63 (NaT) on x86 but 0 (1970-01-01) on arm*.