[AArch64] Tighten up directives tests

Move expected-fail cases from directive-cpu.s to
directive-cpu-err.s. This allows us to remove the 'not' from the
llvm-mc invocation in directive-cpu.s so that this test will fail
in unexpected error cases. It also means that we are not relying
on all stderr coming before any stdout, which seems fragile.

Also make use of CHECK-NEXT to ensure that multiline error messages
really are occuring together.

And add a test to verify that .cpu with an arch version as extension
is rejected.

Differential Revision: https://reviews.llvm.org/D47873

llvm-svn: 335586