[clang] [llvm] [ARM] Adding diagnostics for mcmodel=tiny when used in invalid targets (PR #125643)

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ShashwathiNavada wrote:

@hstk30-hw I have been waiting for approval from @cor3ntin, the PR has been open for over 2 months now, can we go ahead and merge this if it's alright?

https://github.com/llvm/llvm-project/pull/125643