[flang] Fix triple in a couple of driver tests

In https://reviews.llvm.org/D124667, I added tests that check the
generated assembly. I verified the assembly on AArch64 and X86_64, but
the PPC Flang buildbot [1] started failing (i.e. the assembly was not
generic enough).

In order to fix this, I'm changing these tests to be only run on
AAarch64 - that's the architecture that most of public Flang buildbots
use.

I'm hoping that this is straightforward enough and am merging it without
a review.

[1] https://lab.llvm.org/buildbot/#/builders/21/builds/40256