time: Rewrite Now_US() again to ease automatic proving

Currently it struggles with the 64-bit modular to range conversion.
Adding a step in between, a 63-bit modular type, works better.

Change-Id: I32b1a771b18bcc243244cb05636dae6d9f148f90
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/libhwbase/+/501
Tested-by: Ada SPARK <gnatbot@sourcearcade.org>
1 file changed