amd64: FRED support

FRED support as defined starting from the SDM rev. 90, requires a new
'events' entry point to receive user and kernel mode exceptions and
interrupts notifications from the hardware. A minimal asm trampoline is
enough, rest can be implemented in C due to the clean FRED organization
of the event reporting.

The syscall entry is handled by a microptimized assembly path, directly
calling into the amd64_syscall() handler, instead of the generic events
entry point.

Tested by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D55829