RFR(M) 8188165: PPC64: Optimize Unsafe.copyMemory and arraycopy

[Matthew Brandyberry]

This is specific to PPC64LE only.

The emphasis in the proposed code is on minimizing branches. Thus, this 
code makes no attempt to avoid misaligned accesses and each block is 
designed to copy as many elements as possible.

As one data point, this yields as much as a 13x improvement in 
jbyte_disjoint_arraycopy for certain misaligned scenarios.

Bug: https://bugs.openjdk.java.net/browse/JDK-8188165
Webrev: http://cr.openjdk.java.net/~mbrandy/8188165/jdk10/v1/

Thanks,
-Matt