Power configure fixes.

Power configure fixes.

This patch makes the following updates:
  - Update the configure scripts to fix a configure/build issue seen on Power 7
  - Update the ISA 3.0 hwcaps string.
  - Update the ISA 3.0 ifdef value to be consistent with the hwcaps string

The patch was submitted by Will Schmidt.  Patch reviewed and tested by Carl Love
on Power 7, Power 8 LE and BE, ISA 3.0 simulator for LE and BE.

Bugzilla 369175

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16034