Reclaim 16 IDT vectors and make them available for general allocation.

Reclaim vectors 0x20-0x2f by reallocating the IRQ_MOVE_CLEANUP_VECTOR
to vector 0x1f.  This is in the range of vector numbers that is
officially reserved for the CPU (for exceptions), however, the use of
the APIC to generate any vector 0x10 or above is documented, and the
CPU internally can receive any vector number (the legacy BIOS uses INT
0x08-0x0f for interrupts, as messed up as that is.)

Since IRQ_MOVE_CLEANUP_VECTOR has to be alone in the lowest-numbered
priority level (block of 16), this effectively enables us to reclaim
an otherwise-unusable APIC priority level and put it to use.

Since this is a transient kernel-only allocation we can change it at
any time, and if/when there is an exception at vector 0x1f this
assignment needs to be changed as part of OS enabling that new feature.
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <4B4284C6.9030107@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>