[clang] [flang] [mlir] [flang] Add basic -mtune support (PR #95043)

[Alexis Perry-Holby via cfe-commits]

AlexisPerry wrote:

I have lost my upstream commit privileges due to inactivity, so once this is fully approved, could someone merge it on my behalf?  Thank you.

https://github.com/llvm/llvm-project/pull/95043