[llvm] [update_mc_test_checks] Fix llvm-mc invocation on Windows by avoiding echo pipe (PR #179381)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Based on https://github.com/llvm/llvm-project/pull/179381#issuecomment-3844848357 it was my understanding that the test suite still does fail on Windows, and in a way such that the script is not useful on Windows. It also does not seem like there is any plan to get the tests passing on Windows, which I would like to see if we are going to start landing patches for Windows support.

https://github.com/llvm/llvm-project/pull/179381