Commit 281402e0a5 ("arm64: Shave off two instructions in exceptions")
removed the instruction that set the frame pointer (x29) as it appeared
to be unused.
The frame pointer is used in arm64/db_trace.c:db_stack_trace_cmd() when
unwinding state, and hence still needs to be set.
Add back the instruction to save_registers to properly update frame
pointer.
Reported by: andrew
Sponsored by: Arm Ltd
2b8c4137d4