Makefile: Default INDEX_JOBS value to kern.smp.cpus

INDEX_JOBS currently defaults to 2 for the make index target.

MAKE_JOBS_NUMBER in bsd.port.mk currently uses kern.smp.cpus to select
a default. Modify INDEX_JOBS to match.

Nominal and repeatable performance improvements of ~20-30% were observed.

CR: D381
Reviewed by: swills, bdrewery
Approved by: portmgr