In environments where time_t is 32 bits, including the 32-bit library
build on amd64, the overflow being tested for cannot happen, and gcc
complains with -Wtype-limits, causing the gcc build to fail. Work
around this by tweaking the saturation test to test for the limit values
too. This retains a useless but harmless comparison for 32-bit time_t.
Fixes: e3799530b3ba ("sys/time: Add saturating sbt conversions")