PowerNV: Send SIGILL on HEA illegal instruction exception

Currently Hypervisor Emulation Assistance interrupt is unhandled.
Executing an undefined instruction in userland triggers kernel panic.
Handle this the same way as Facility Unavailable Interrupt - send
SIGILL signal to userspace.

Submitted by: Michal Stanek <mst@semihalf.com>
Obtained from: Semihalf
Reviewed by: nwhitehorn, pdk@semihalf.com, wma
Sponsored by: IBM, QCM Technologies
Differential revision: https://reviews.freebsd.org/D14437