gas/ppc: Additional tests for DFP instructions

I noticed that some of the Power6 DFP instructions were not covered by
the assembler tests.  I've added a new test file which I believe
covers all the DFP Power6 instructions.

The existing gas/testsuite/gas/ppc/power6.d test is called:

  POWER6 tests (includes DFP and Altivec)

And does cover some of the DFP instructions.  But, given the number of
additional instructions I'm adding I opted to add a whole new test
file.  I've left the original power6.d unchanged, so there is now some
overlap, but I don't think that should hurt much.