fix up r353340, don't assume that fcmpset has strong semantics

fcmpset can have two kinds of semantics, weak and strong.
For practical purposes, strong semantics means that if fcmpset fails
then the reported current value is always different from the expected
value. Weak semantics means that the reported current value may be the
same as the expected value even though fcmpset failed. That's a so
called "sporadic" failure.

I originally implemented atomic_cas expecting strong semantics, but many
platforms actually have weak one.

Reported by: pkubaj (not confirmed if same issue)
Discussed with: kib, mjg
MFC after: 19 days
X-MFC with: r353340