[clang] [Clang] Amend SME attributes with support for ZT0. (PR #77941)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

@sdesmalen-arm This appears to be failing on some buildbots: https://lab.llvm.org/buildbot/#/builders/176/builds/8232
```
llvm-lit: /home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/llvm/llvm/utils/lit/lit/TestingConfig.py:152: fatal: unable to parse config file '/home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/stage2/runtimes/runtimes-bins/compiler-rt/unittests/lit.common.unit.configured', traceback: Traceback (most recent call last):
  File "/home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/llvm/llvm/utils/lit/lit/TestingConfig.py", line 140, in load_from_path
    exec(compile(data, path, "exec"), cfg_globals, None)
  File "/home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/stage2/runtimes/runtimes-bins/compiler-rt/unittests/lit.common.unit.configured", line 23
    config.aarch64_sme =
                        ^
SyntaxError: invalid syntax
```

https://github.com/llvm/llvm-project/pull/77941