[LLDB] [python test] Builder::build does not take into account architecture param

from this comment: https://github.com/llvm/llvm-project/pull/168273#discussion_r2532211198 .

[Builder::getBuildCommand](https://github.com/llvm/llvm-project/blob/main/lldb/packages/Python/lldbsuite/test/builders/builder.py#L274C9-L274C24)
accepts architecture as param
and calls to
```
self.getArchCFlags(architecture),
self.getArchSpec(architecture),
```

to prepare a build command.
which both under the hood call to [getTriple](https://github.com/llvm/llvm-project/blob/main/lldb/packages/Python/lldbsuite/test/builders/builder.py#L27) which is not taking into account arch parameter and return triple from `configuration` (which is apparently also None).

And so the command used to build a target does not take into account architecture when its value is provided to `build` and `getBuildCommand`.

Start in lldb/packages/Python/lldbsuite/test/builders/builder.py at Builder::getBuildCommand, getArchCFlags, getArchSpec, and getTriple. Trace how the architecture argument reaches the generated build command, then verify with the relevant LLDB Python test that a supplied architecture is reflected in the command.

Written by the indexing model from the issue text.