cmd/internal/obj: rework gclocals handling

The compiler handled gcargs and gclocals LSyms unusually.
It generated placeholder symbols (makefuncdatasym),
filled them in, and then renamed them for content-addressability.
This is an important binary size optimization;
the same locals information occurs over and over.

This CL continues to treat these LSyms unusually,
but in a slightly more explicit way,
and importantly for concurrent compilation,
in a way that does not require concurrent
modification of Ctxt.Hash.

Instead of creating gcargs and gclocals in the usual way,
by creating a types.Sym and then an obj.LSym,
we add them directly to obj.FuncInfo,
initialize them in obj.InitTextSym,
and deduplicate and add them to ctxt.Data at the end.
Then the backend's job is simply to fill them in
and rename them appropriately.

Updates #15756

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       38.7MB ± 0%  -0.22%  (p=0.016 n=5+5)
Unicode         29.8MB ± 0%       29.8MB ± 0%    ~     (p=0.690 n=5+5)
GoTypes          113MB ± 0%        113MB ± 0%  -0.24%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.24GB ± 0%  -0.39%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.2MB ± 0%  -0.43%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       31.7MB ± 0%  -0.22%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       77.6MB ± 0%  -0.80%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       26.3MB ± 0%  -0.85%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       41.9MB ± 0%  -1.04%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          378k ± 0%         377k ± 1%    ~     (p=0.151 n=5+5)
Unicode           321k ± 1%         321k ± 0%    ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%  -0.47%  (p=0.016 n=5+5)
SSA              9.71M ± 0%        9.67M ± 0%  -0.33%  (p=0.008 n=5+5)
Flate             233k ± 1%         232k ± 1%    ~     (p=0.151 n=5+5)
GoParser          316k ± 0%         315k ± 0%  -0.49%  (p=0.016 n=5+5)
Reflect           979k ± 0%         972k ± 0%  -0.75%  (p=0.008 n=5+5)
Tar               250k ± 0%         247k ± 1%  -0.92%  (p=0.008 n=5+5)
XML               392k ± 1%         389k ± 0%  -0.67%  (p=0.008 n=5+5)

Change-Id: Idc36186ca9d2f8214b5f7720bbc27b6bb22fdc48
Reviewed-on: https://go-review.googlesource.com/40697
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/fmt_test.go

@@ -654,7 +654,6 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/types.EType %v":             "",
 	"cmd/internal/src.Pos %s":                         "",
 	"cmd/internal/src.Pos %v":                         "",
-	"cmd/internal/src.XPos %v":                        "",
 	"error %v":                                        "",
 	"float64 %.2f":                                    "",
 	"float64 %.3f":                                    "",