time: switch to fences for siginfo_recvd

This effectively reverts
6e824f3713011 ("time: siginfo_recvd needs to be marked volatile")
because it was actually wrong. Switch to C11 signal fence, which
provides a compiler barrier that will do the right thing.

Reported by: kib
Reviewed by: kib (slightly earlier version)
Differential Revision: https://reviews.freebsd.org/D45574