[Utils] Replace llc with cat for tests

Make the update_llc_test_checks script test independant of llc behavior
by using cat with static files to simulate llc output.

This allows changing llc without breaking the script test case.

The update script is executed in a temporary directory, so the
llc-generated assembly files are copied there. %T is deprecated, but it
allows copying a file with a predictable filename.

Differential Revision: https://reviews.llvm.org/D110143