cmd/go: enable concurrent backend compilation by default

It can be disabled by setting the environment variable
GO19CONCURRENTCOMPILATION=0, or with -gcflags=-c=1.

Fixes #15756.

Change-Id: I7acbf16330512b62ee14ecbab1f46b53ec5a67b6
Reviewed-on: https://go-review.googlesource.com/41820
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/go/internal/work/build.go

@@ -144,7 +144,7 @@ See also: go install, go get, go clean.
 	`,
 }
 
-const concurrentGCBackendCompilationEnabledByDefault = false
+const concurrentGCBackendCompilationEnabledByDefault = true
 
 func init() {
 	// break init cycle