[lldb][test] Unify test infrastructure for checking CPU features (#153914)

This addresses limitations in our testing infrastructure for checking
CPU features. Before this
* `getCPUInfo()` was Linux-only, and the
* `@skipUnlessFeature` decorator was Darwin-only and did not consider
the remote (on device) testing use case.

Introduce `CPUFeature` class as an abstraction to hide the
platform-specific implementations to check for CPU features. Unify local
(on host) and remote (on device) test execution by always going through
`test.run_platform_command()` which uses LLDB's `platform shell <cmd>`
command.

Potential future cleanups: I think `@skipUnlessFeature` decorator could
be used in place of code like this:
```
if not self.isAArch64SME():
    self.skipTest("SME must be present.")
if not self.isAArch64SME2():
    self.skipTest("SME2 must be present.")
```

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