cmd/go: move gc concurrency level computation near gcflags

So after constructing "args" variable, "gcflags" is not used anywhere.
It makes the code easier to maintain, and prevent subtle bug like #48490.

Change-Id: I41653536480880a8a6f9fbf6cfa8a461b6fb3208
Reviewed-on: https://go-review.googlesource.com/c/go/+/351849
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>

Author: cuonglm

src/cmd/go/internal/work/gc.go

@@ -156,6 +156,10 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 			}
 		}
 	}
+	// Add -c=N to use concurrent backend compilation, if possible.
+	if c := gcBackendConcurrency(gcflags); c > 1 {
+		gcflags = append(gcflags, fmt.Sprintf("-c=%d", c))
+	}
 
 	args := []interface{}{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", a.trimpath(), defaultGcFlags, gcflags}
 	if p.Internal.LocalPrefix != "" {
@@ -181,11 +185,6 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 		args = append(args, "-asmhdr", objdir+"go_asm.h")
 	}
 
-	// Add -c=N to use concurrent backend compilation, if possible.
-	if c := gcBackendConcurrency(gcflags); c > 1 {
-		args = append(args, fmt.Sprintf("-c=%d", c))
-	}
-
 	for _, f := range gofiles {
 		f := mkAbs(p.Dir, f)