A64: Implement BRK

Currently, we can just implement this as part of the exception
interface, similar to how it's done for the A32 interface with BKPT.

include/dynarmic/A64/config.h

@@ -38,6 +38,8 @@ enum class Exception {
     SendEventLocal,
     /// A YIELD instruction was executed.
     Yield,
+    /// A BRK instruction was executed.
+    Breakpoint,
 };
 
 enum class DataCacheOperation {