cmd/go: honor user-specified -c=N

User-specified '-c=N' options should take precedence over the default one generated
by cmd/go.

Change-Id: I9021440b4a4286f8d6f92acb745a777cf234a82b
Reviewed-on: https://go-review.googlesource.com/c/go/+/465095
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -169,7 +169,7 @@ 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))
+		defaultGcFlags = append(defaultGcFlags, fmt.Sprintf("-c=%d", c))
 	}
 
 	args := []any{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", a.trimpath(), defaultGcFlags, gcflags}

src/cmd/go/testdata/script/build_gcflags_order.txt

@@ -4,6 +4,8 @@
 cd m
 go build -n -gcflags=-lang=go1.17
 stderr ' -lang=go1.16.* -lang=go1.17'
+! go build -gcflags='-c 0'
+stderr 'compile: -c must be at least 1, got 0'
 
 -- m/go.mod --
 module example.com