RFR(M) JDK-8181809 PPC64: Leverage mtfprd/mffprd on POWER8

[Matthew Brandyberry]

This is a PPC-specific hotspot optimization that leverages the 
mtfprd/mffprd instructions for for movement between general purpose and 
floating point registers (rather than through memory).  It yields a ~35% 
improvement measured via a microbenchmark. Please review: Bug: 
https://bugs.openjdk.java.net/browse/JDK-8181809 Webrev: 
http://cr.openjdk.java.net/~gromero/8181809/v1/ Thanks, Matt