runtime: delete gentraceback

Printing is the only remaining functionality of gentraceback. Move
this into the traceback printing code and eliminate gentraceback. This
lets us simplify the logic, which fixes at least one minor bug:
previously, if inline unwinding pushed the total printed count over
_TracebackMaxFrames, we would print extra frames and then fail to
print "additional frames elided".

The cumulative performance effect of the series of changes starting
with "add a benchmark of Callers" (CL 472956) is:

goos: linux
goarch: amd64
pkg: runtime
cpu: Intel(R) Xeon(R) CPU E5-2690 v3 @ 2.60GHz
                       │  baseline   │              unwinder               │
                       │   sec/op    │   sec/op     vs base                │
Callers/cached-48        1.464µ ± 1%   1.684µ ± 1%  +15.03% (p=0.000 n=20)
Callers/inlined-48       1.391µ ± 1%   1.536µ ± 1%  +10.42% (p=0.000 n=20)
Callers/no-cache-48      10.50µ ± 1%   11.11µ ± 0%   +5.82% (p=0.000 n=20)
StackCopyPtr-48          88.74m ± 1%   81.22m ± 2%   -8.48% (p=0.000 n=20)
StackCopy-48             80.90m ± 1%   70.56m ± 1%  -12.78% (p=0.000 n=20)
StackCopyNoCache-48      2.458m ± 1%   2.209m ± 1%  -10.15% (p=0.000 n=20)
StackCopyWithStkobj-48   26.81m ± 1%   25.66m ± 1%   -4.28% (p=0.000 n=20)
geomean                  518.8µ        512.9µ        -1.14%

The performance impact of intermediate CLs in this sequence varies a
lot as we went through many refactorings. The slowdown in Callers
comes primarily from the introduction of unwinder because that doesn't
get inlined and results in somewhat worse code generation in code
that's extremely hot in those microbenchmarks. The performance gains
on stack copying come mostly from replacing callbacks with direct use
of the unwinder.

Updates #54466.
Fixes #32383.

Change-Id: I4970603b2861633eecec30545e852688bc7cc9a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/468301
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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