arm64: fix stack unwinding past exception handlers

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

sys/arm64/arm64/exception.S

@@ -73,6 +73,7 @@ __FBSDID("$FreeBSD$");
 .endm
 
 .macro	save_registers el
+	add	x29, sp, #(TF_SIZE)
 .if \el == 0
 #if defined(PERTHREAD_SSP)
 	/* Load the SSP canary to sp_el0 */