RFR(S) JDK-8181810 PPC64: Leverage extrdi for bitfield extract

[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