cmd/internal/objfile: emit trailing tab outside of Disasm.Decode
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.