tests,arch-arm: Add guards for ARM-specific test

The nightly tests, https://jenkins.gem5.org/job/nightly/219/, were
failing with the following error when running build/NULL/unittests.opt:

```
[ENUMDECL] m5.objects.ArmSystem, ArmExtension -> NULL/enums/ArmExtension.hh
terminate called after throwing an instance of 'pybind11::error_already_set'
  what():  ModuleNotFoundError: No module named 'm5.objects.ArmSystem'
At:
  <frozen importlib._bootstrap>(973): _find_and_load_unlocked
  <frozen importlib._bootstrap>(991): _find_and_load
  <frozen importlib._bootstrap>(1014): _gcd_import
  /usr/lib/python3.8/importlib/__init__.py(127): import_module
  build_tools/enum_hh.py(58): <module>
Aborted (core dumped)
scons: *** [build/NULL/enums/ArmExtension.hh] Error 134
```

The reason for this is the 'aapcs64.test' now transitively includes the
'ArmExtension' enum via this commit:
https://gem5-review.googlesource.com/c/public/gem5/+/59471.

As this test now only works with the ARM ISA, a guard has been included.
As noted in the comment, GTest does not have the 'tags' parameter so the
'TARGET_ISA' environment variable was used. This will need updated when
the multi-isa code is incorporated.

Change-Id: I2793094bf7c813afd97933750332fa3f3b7bd8dd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59569
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed