arch/x86_64: do not modify CR8 in interrupt path

Currently, the interrupt service code manually raises the CPU task
priority to the priority level of the vector being serviced to defer
any lower-priority interrupts. This is unnecessary; the local APIC
is aware that an interrupt is in-service and accounts for its priority
when deciding whether to issue an overriding interrupt to the CPU.

Signed-off-by: Charles E. Youse <charles@gnuless.org>