cmd/compile: use a bytes.Buffer to format symbols

No significant compiler speed changes,
but some minor memory savings.

Passes toolstash-check.

name        old alloc/op      new alloc/op      delta
Template         36.3MB ± 0%       36.2MB ± 0%  -0.25%  (p=0.008 n=5+5)
Unicode          28.3MB ± 0%       28.2MB ± 0%  -0.16%  (p=0.008 n=5+5)
GoTypes           122MB ± 0%        121MB ± 0%  -0.39%  (p=0.008 n=5+5)
Compiler          568MB ± 0%        566MB ± 0%  -0.21%  (p=0.008 n=5+5)
SSA              1.95GB ± 0%       1.95GB ± 0%  -0.08%  (p=0.008 n=5+5)
Flate            22.8MB ± 0%       22.8MB ± 0%  -0.21%  (p=0.008 n=5+5)
GoParser         28.0MB ± 0%       27.9MB ± 0%  -0.38%  (p=0.008 n=5+5)
Reflect          78.6MB ± 0%       78.3MB ± 0%  -0.33%  (p=0.008 n=5+5)
Tar              34.1MB ± 0%       34.1MB ± 0%  -0.19%  (p=0.008 n=5+5)
XML              44.3MB ± 0%       44.2MB ± 0%  -0.19%  (p=0.008 n=5+5)
[Geo mean]       79.9MB            79.7MB       -0.24%

name        old allocs/op     new allocs/op     delta
Template           363k ± 0%         359k ± 0%  -1.21%  (p=0.008 n=5+5)
Unicode            329k ± 0%         326k ± 0%  -0.90%  (p=0.008 n=5+5)
GoTypes           1.28M ± 0%        1.25M ± 0%  -2.62%  (p=0.008 n=5+5)
Compiler          5.39M ± 0%        5.31M ± 0%  -1.45%  (p=0.008 n=5+5)
SSA               18.1M ± 0%        17.9M ± 0%  -0.78%  (p=0.008 n=5+5)
Flate              228k ± 0%         226k ± 0%  -0.97%  (p=0.008 n=5+5)
GoParser           295k ± 0%         288k ± 0%  -2.26%  (p=0.008 n=5+5)
Reflect            949k ± 0%         932k ± 0%  -1.74%  (p=0.008 n=5+5)
Tar                336k ± 0%         332k ± 0%  -1.12%  (p=0.008 n=5+5)
XML                417k ± 0%         413k ± 0%  -1.10%  (p=0.008 n=5+5)
[Geo mean]         818k              806k       -1.42%

Change-Id: Ibdb94650a761edec17d8eba0abdfb70a8a495da9
Reviewed-on: https://go-review.googlesource.com/c/go/+/222920


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>