cmd/internal/obj/{arm,s390x}: make return jump print nicer

cmd/internal/obj/{arm,s390x}: make return jump print nicer

When a function with non-zero frame size makes a return jump
(RET target), it assembles to, conceptually,

MOV (SP), LR
ADD $framesize, SP
JMP target

We did not clear some fields in the first instruction's Prog.To,
causing it printed like (on ARM)

MOVW.P	4(R13), (R14)(R14)(REG)

Clear the fields to make it print nicer.

Change-Id: I180901aeea41f1ff287d7c6034a6d69005927744
Reviewed-on: https://go-review.googlesource.com/c/go/+/264343
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>