Fix the scratch register used for arm64 stack probing (dotnet/coreclr#22313)

Fix the scratch register used for arm64 stack probing (dotnet/coreclr#22313)

* Fix the scratch register used for arm64 stack probing

In some circumstances, genFnProlog might pick a callee-saved
register to be the initReg. This doesn't work for stack probing
since we might need a scratch register before we save the
callee-save regs. So, always use R9 (REG_SCRATCH) instead.

Fixes dotnet/coreclr#22284

* Formatting

Commit migrated from https://github.com/dotnet/coreclr/commit/53e367b7f9a2ca799e0510f9420567848e7c0e8b