cmd/builder: automatically choose nacltest.bash for nacl- builds

This is a recapitulation of CL 114780043.

This change extends the buildCmd logic to automatically choose the
nacltest.bash build script if the builder key represents a nacl job.

This is a change I have been carrying for over year now which I have
to remember to reapply every time I rebuild the nacl-arm builder.

Change-Id: I3e86a3e62671cda6feb318bb4b312f5462436f27
Reviewed-on: https://go-review.googlesource.com/4261
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/builder/doc.go

@@ -36,6 +36,10 @@ Optional flags:
   -rev=N: Build revision N and exit
 
   -cmd="./all.bash": Build command (specify absolute or relative to go/src)
+    This flag is overridden in the following conditions:
+
+    - If the build key ends in -race, then race.bash or race.bat will be chosen.
+    - If the build key begins with nacl, then nacltest.bash will be chosen.
 
   -v: Verbose logging
 

cmd/builder/main.go

@@ -74,6 +74,7 @@ var (
 	releaseRe   = regexp.MustCompile(`^release\.r[0-9\-.]+`)
 	allCmd      = "all" + suffix
 	makeCmd     = "make" + suffix
+	naclCmd     = "nacltest.bash" // nacl support is linux only
 	raceCmd     = "race" + suffix
 	cleanCmd    = "clean" + suffix
 	suffix      = defaultSuffix()
@@ -361,10 +362,15 @@ func (b *Builder) builderEnv(name string) (builderEnv, error) {
 // buildCmd returns the build command to invoke.
 // Builders which contain the string '-race' in their
 // name will override *buildCmd and return raceCmd.
+// Builders which start with "nacl-" will override
+// *buildCmd and return naclCmd.
 func (b *Builder) buildCmd() string {
 	if strings.Contains(b.name, "-race") {
 		return raceCmd
 	}
+	if strings.HasPrefix(b.name, "nacl-") {
+		return naclCmd
+	}
 	return *buildCmd
 }