[llvm] Reserve R9 on armv6 iOS before 3.0 (PR #150835)

[via llvm-commits]

Un1q32 wrote:

All armv6 iOS devices were armv6k, and `clang -target armv6-apple-ios2` uses armv6k, but `llc -mtriple=armv6-apple-ios2` doesn't so catching armv6 should help anyone running llc like that.

https://github.com/llvm/llvm-project/pull/150835