arm64: fix stack unwinding past exception handlers

Commit 281402e0a563 ("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