cmd/internal/objfile: reuse tabwriter across symbols when disassembling
cmd/internal/objfile: reuse tabwriter across symbols when disassembling

Since the tabwriter is flushed at every symbol,
it can be re-used with no impact on the output.

This cuts allocated space when objdump-ing
the compiler by almost 40%,
and enables further upcoming improvements.

It also speeds up objdump.

name            old time/op       new time/op       delta
ObjdumpCompile        9.22s ± 3%        8.77s ± 3%   -4.79%  (p=0.000 n=10+9)

Change-Id: Ief114d6c2680a4e762b5f439d3ca8dc7a89b9b27
Reviewed-on: https://go-review.googlesource.com/106978
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,josharian/go,nawawi/go,josharian/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,josharian/go,nawawi/go,golang/go,CAFxX/go,akutz/go,golang/go,golang/go,cloudflare/go,josharian/go,josharian/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,josharian/go,golang/go,cloudflare/go,josharian/go,akutz/go,josharian/go,golang/go,akutz/go,CAFxX/go,akutz/go,golang/go,golang/go,nawawi/go,akutz/go,akutz/go,akutz/go