Add x86 specific APIs 'lapic_ipi_alloc()' and 'lapic_ipi_free()' to allow IPI
vectors to be dynamically allocated. This allows kernel modules like vmm.ko
to allocate unique IPI slots when loaded (as opposed to hard allocating one
or more vectors).

Also, reorganize the fixed IPI vectors to create a contiguous space for
dynamic IPI allocation.

Reviewed by: kib, jhb
Differential Revision: https://reviews.freebsd.org/D2042