understands FPU hardware enough to catch SIGFPE and unmask exceptions
in control word, then it may as well properly handle return from
SIGFPE without causing an infinite loop of #MF exceptions due to
faulting instruction restart, when needed.
Clearing exceptions causes information loss for handlers which do
understand FPU hardware, and struct siginfo si_code member cannot be
considered adequate replacement for en_sw content due to translation.
Supposed reason for clearing the exceptions, which is IRQ13 handling
oddities, were never applicable to amd64.
Note: this change will be merged to i386 kernel as well, since we do
not support IRQ13 delivery of #MF notifications for some time.
Requested by: bde
MFC after: 1 week
dfa8a51288