PowerPC, fix for conv_f16_to_double xscvhpdp assembler code

PowerPC, fix for conv_f16_to_double xscvhpdp assembler code

The previous commit:

  commit eb82a294573d15c1be663673d55b559a82ca29d3
  Author: Julian Seward <jseward@acm.org>
  Date:   Tue Nov 10 21:10:48 2020 +0100

      Add a missing ifdef, whose absence caused build breakage on non-POWER targets.

fixed the compile issue in conv_f16_to_double() where non-Power platforms
do not support the power xscvhpdp assembly instructions.  The instruction
is supported by ISA 3.0 platforms.  Older Power platforms still fail to
compile with the assembly instruction.  This patch fixes the if def for
power systems that do not support ISA 3.0.