This cuts the allocated space while executing

go tool objdump -S `go tool -n compile`

by over 10%.

It also speeds it up slightly:

name              old time/op       new time/op       delta
ObjdumpSCompiler        9.03s ± 1%        8.88s ± 1%  -1.59%  (p=0.000 n=20+20)

Updates #24725

Change-Id: Ic6ef8e273ede589334ab6e07099ac2e5bdf990c9
Reviewed-on: https://go-review.googlesource.com/106798
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>