cmd/internal/dwarf: remove global encbuf

cmd/internal/dwarf: remove global encbuf

The global encbuf helped avoid allocations.
It is incompatible with a concurrent backend.
To avoid a performance regression while removing it,
introduce two optimizations.
First, re-use a buffer in dwarf.PutFunc.
Second, avoid a buffer entirely when the int
being encoded fits in seven bits, which is about 75%
of the time.

Passes toolstash-check.

Updates #15756

name       old alloc/op    new alloc/op    delta
Template      40.6MB ± 0%     40.6MB ± 0%  -0.08%  (p=0.001 n=8+9)
Unicode       29.9MB ± 0%     29.9MB ± 0%    ~     (p=0.068 n=8+10)
GoTypes        116MB ± 0%      116MB ± 0%  +0.05%  (p=0.043 n=10+9)
SSA            864MB ± 0%      864MB ± 0%  +0.01%  (p=0.010 n=10+9)
Flate         25.8MB ± 0%     25.8MB ± 0%    ~     (p=0.353 n=10+10)
GoParser      32.2MB ± 0%     32.2MB ± 0%    ~     (p=0.353 n=10+10)
Reflect       80.2MB ± 0%     80.2MB ± 0%    ~     (p=0.165 n=10+10)
Tar           27.0MB ± 0%     26.9MB ± 0%    ~     (p=0.143 n=10+10)
XML           42.8MB ± 0%     42.8MB ± 0%    ~     (p=0.400 n=10+9)

name       old allocs/op   new allocs/op   delta
Template        398k ± 0%       397k ± 0%  -0.20%  (p=0.002 n=8+9)
Unicode         320k ± 0%       321k ± 1%    ~     (p=0.122 n=8+10)
GoTypes        1.16M ± 0%      1.17M ± 0%    ~     (p=0.053 n=10+9)
SSA            7.65M ± 0%      7.65M ± 0%    ~     (p=0.122 n=10+8)
Flate           240k ± 1%       240k ± 1%    ~     (p=0.243 n=10+9)
GoParser        322k ± 1%       322k ± 1%    ~     (p=0.481 n=10+10)
Reflect        1.00M ± 0%      1.00M ± 0%    ~     (p=0.211 n=9+10)
Tar             256k ± 0%       255k ± 1%    ~     (p=0.052 n=10+10)
XML             400k ± 1%       400k ± 0%    ~     (p=0.631 n=10+10)

Change-Id: Ia39d9de09232fdbfc9c9cec14587bbf6939c9492
Reviewed-on: https://go-review.googlesource.com/38713
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>