sys/time: appease gcc -Wtype-limits

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 ifdef'ing out the saturation code on i386.

Reviewed by: imp, jfree
Discussed with: markj
Fixes: e3799530b3ba ("sys/time: Add saturating sbt conversions")
Differential Revision: https://reviews.freebsd.org/D56369