[8u] RFR (M) 8181809 PPC64: Leverage mtfprd/mffprd on POWER8

[Matthew Brandyberry]

Hi,

Please review this backport of 8181809 for jdk8u.

It applies cleanly to jdk8u except for the lack of C1 support on PPC in 
8u -- thus those changes are omitted here.

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.

webrev       :http://cr.openjdk.java.net/~mbrandy/8181809/jdk8u/v1

bug          :https://bugs.openjdk.java.net/browse/JDK-8181809

review thread:http://mail.openjdk.java.net/pipermail/hotspot-dev/2017-June/027226.html


Thank you.
-Matt