time: switch to fences for siginfo_recvd

[kevans]

Summary

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

[kib]

usr.bin/time/time.c

130

I think that the right code is

local_siginfo_recvd = siginfo_recvd;
atomic_signal_fence(acq);
if (local siginfo_recvd) {
  ....

[Fence should come after reading the flag variable]