[llvm] [clang] [llvm][ARM] Emit MVE .arch_extension after .fpu directive if it does not include MVE features (PR #71545)

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simpal01 wrote:

> I think the commit title would make more sense at a glance if it was saying what is being done instead of what's being fixed. How about:
> 
> ```
> [llvm][ARM] Emit MVE .arch_extension after .fpu directive if it does not include MVE features
> ```
> 
> Then the commit message is fine as is, and tells the reader why this was done and what it fixes.

Done

https://github.com/llvm/llvm-project/pull/71545