[MC] Make llvm-mc fail cleanly on invalid output asm variant. Fixes PR28488. Reviewers: rnk, majnemer Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D25834 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285616 91177308-0d34-0410-b5e6-96231b3b80d8