[MC][test] Fix non-portable GNU diff option

Summary: This patch replaces the non-portable GNU diff option `--strip-trailing-cr` with the POSIX `-b` option in two test files.

Reviewers: daltenty, jasonliu

Reviewed By: daltenty

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72745