Add benchmark for indirect invocation.
Add benchmark for indirect invocation.

Note that the indirection costs quite a bit.

BenchmarkASM-4        	1000000000	         2.25 ns/op
BenchmarkASMIndirect-4	300000000	         4.13 ns/op
BenchmarkGeneric-4    	500000000	         3.38 ns/op

hideo55/go-popcount