[lldb] Fix arm breakages from D130985

The kernel was rejecting sizeof(struct GPR) as it was not a multiple of
8. Add a padding field to fix that.

One also wonders whether "cpsr" is right register name for aarch64.

lldb/source/Plugins/Process/Utility/RegisterInfoPOSIX_arm64.h

@@ -47,6 +47,7 @@ public:
     uint64_t sp;    // x31
     uint64_t pc;    // pc
     uint32_t cpsr;  // cpsr
+    uint32_t pad;
   };
   LLVM_PACKED_END