[clang] [AArch64][clang] Improve -march= error message with many feature flags (PR #197441)

[Jonathan Thackray via cfe-commits]

jthackray wrote:

> Sure, happy to do a follow-up patch to this one.

@smithp35 Follow-up patch in https://github.com/llvm/llvm-project/pull/197640.

https://github.com/llvm/llvm-project/pull/197441