cmd/compile: move Text.From.Sym initialization earlier

cmd/compile: move Text.From.Sym initialization earlier

The initialization of an ATEXT Prog's From.Sym
can race with the assemblers in a concurrent compiler.
CL 40254 contains an initial, failed attempt to
fix that race.

This CL takes a different approach: Rather than
expose an API to initialize the Prog,
expose an API to initialize the Sym.

The initialization of the Sym can then be
moved earlier in the compiler, avoiding the race.

The growth of gc.Func has negligible
performance impact; see below.

Passes toolstash -cmp.

Updates #15756

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       38.8MB ± 0%    ~     (p=0.968 n=9+10)
Unicode         29.8MB ± 0%       29.8MB ± 0%    ~     (p=0.684 n=10+10)
GoTypes          113MB ± 0%        113MB ± 0%    ~     (p=0.912 n=10+10)
SSA             1.25GB ± 0%       1.25GB ± 0%    ~     (p=0.481 n=10+10)
Flate           25.3MB ± 0%       25.3MB ± 0%    ~     (p=0.105 n=10+10)
GoParser        31.7MB ± 0%       31.8MB ± 0%  +0.09%  (p=0.016 n=8+10)
Reflect         78.3MB ± 0%       78.2MB ± 0%    ~     (p=0.190 n=10+10)
Tar             26.5MB ± 0%       26.6MB ± 0%  +0.13%  (p=0.011 n=10+10)
XML             42.4MB ± 0%       42.4MB ± 0%    ~     (p=0.971 n=10+10)

name       old allocs/op     new allocs/op     delta
Template          378k ± 1%         378k ± 0%    ~     (p=0.315 n=10+9)
Unicode           321k ± 1%         321k ± 0%    ~     (p=0.436 n=10+10)
GoTypes          1.14M ± 0%        1.14M ± 0%    ~     (p=0.079 n=10+9)
SSA              9.70M ± 0%        9.70M ± 0%  -0.04%  (p=0.035 n=10+10)
Flate             233k ± 1%         234k ± 1%    ~     (p=0.529 n=10+10)
GoParser          315k ± 0%         316k ± 0%    ~     (p=0.095 n=9+10)
Reflect           980k ± 0%         980k ± 0%    ~     (p=0.436 n=10+10)
Tar               249k ± 1%         250k ± 0%    ~     (p=0.280 n=10+10)
XML               391k ± 1%         391k ± 1%    ~     (p=0.481 n=10+10)

Change-Id: I3c93033dddd2e1df8cc54a106a6e615d27859e71
Reviewed-on: https://go-review.googlesource.com/40496
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>