cmd/internal/obj: make asmidx error less cryptic

cmd/internal/obj: make asmidx error less cryptic

It's still pretty cryptic, but at least now instead of printing

    asm: asmidx: bad address 0/2067/2068

it will print

    asm: asmidx: bad address 0/BX/SP

Change-Id: I1c73c439c94c5b9d3039728db85102a818739db9
Reviewed-on: https://go-review.googlesource.com/c/go/+/611256
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Austin Clements <austin@google.com>