[llvm] Mips/X86: Use reportFatalUsageError for subtarget construction errors (PR #142028)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **May 30, 5:58 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/142028).


https://github.com/llvm/llvm-project/pull/142028