Disasm.Decode currently always appends a tab to the formatted instruction,
although not to any relocations after it.

Decode has two clients: objdump and pprof.
pprof emits plain text, so it would be better not to have a trailing tab.
objdump wants the trailing tab for text/tabwriter,
but it is easy to add that to the Fprintf call.

Shifting the responsibility for the trailing tab to the caller
simplifies the code, increases correctness, and slightly improves
performance by reducing and coalescing string concatenations.

Change-Id: I0c85518ee185949e385de819e2e703bce757eba9
Reviewed-on: https://go-review.googlesource.com/106983
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>