mock: simplify forcearch code

[praiskup]

Mock now automatically maps the target build architecture directly to the
appropriate QEMU user-static binary variant for forcearch builds. For
example, fedora-43-riscv64 is mapped to /usr/bin/qemu-riscv64-static (see
the architecture string matches). Mock config contributors no longer need to
modify Mock code to add support for new architectures (if these architecture
specifiers match).

[gemini-code-assist]

Code Review

This pull request simplifies the forcearch implementation by removing the need for most explicit architecture mappings in the configuration. Instead, it dynamically constructs the QEMU binary path from the architecture name, falling back to this convention if a specific mapping isn't found. This is a solid improvement for maintainability and makes adding new architectures much easier.

My review focuses on two main points:

1. The lack of tests for this significant logic change. I've added a comment in mock/py/mock.py about this.
2. A minor clarification in the new release notes to avoid potential user confusion.

Overall, the change is good, but adding tests is crucial to ensure its correctness and prevent future regressions.

[xsuchy]

+1