x86-64-v2 Fix, main branch (2023.08.17.)

Made the code check explicitly whether the compiler understands -march=x86-64-v2. Because as it turns out (see !1023 (comment 7027962)), older versions of GCC and Clang do not. 😦 Versions that would still be good to be able to use with AtlasCMake.

I considered for a moment to teach _add_flag(...) how to test every flag whether it would be valid, but because we use _add_flag(...) for all the different languages at the moment, that would've been a much bigger update. 🤔