Add an efficient implementation of the "count trailing zeros" operation

There are a number of possible uses for an efficient "count trailing zero" operation. By efficient we mean access to the underlying hardware support; supported hardware architectures generally provide an efficient implementation, e.g. a single instruction or a short instruction sequence.

One obvious use is in the BitMap search functions, which are presently performing that operation using a for-loop to iterate over the bits in the word of interest. See JDK-6735527.