[PowerPC] Reset the baseline for ppc64le to be equivalent to pwr8

[PowerPC] Reset the baseline for ppc64le to be equivalent to pwr8

Test by Nemanja Ivanovic.

Since ppc64le implies POWER8 as a minimum, it makes sense that the
same features are included. Since the pwr8 processor model will likely
be getting new features until the implementation is complete, I
created a new list to add these updates to. This will include them in
both pwr8 and ppc64le.

Furthermore, it seems that it would make sense to compose the feature
lists for other processor models (pwr3 and up). Per discussion in the
review, I will make this change in a subsequent patch.

In order to test the changes, I've added an additional run step to
test cases that specify -march=ppc64le -mcpu=pwr8 to omit the -mcpu
option. Since the feature lists are the same, the behaviour should be
unchanged.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227053 91177308-0d34-0410-b5e6-96231b3b80d8