Add ADAPTIVE_GIANT to GENERIC on i386, with the intent of making it
a standard configuration similar to [NO_]ADAPTIVE_MUTEXES. This
feature causes Giant to be included in the set of mutexes adaptively
spun on. It appears to have a positive effect on performance on SMP
across several workloads, including measurements of a 16% improvement
on buildworld, and 30%+ improvement for MySQL using the supersmack
benchmark with Giant over the network stack; a 6% improvement without
Giant on the network stack (as a result of less giant contention).