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

[Alexander Richardson via llvm-commits]

arichardson wrote:

One option I've been thinking about for a while is to use the internal shell helpers in the update_*_checks to avoid portability issues.

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