test: add support for build tags.

This enables a few tests that were only executed
unconditionnally.

R=rsc, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/7103051

test/fixedbugs/bug385_32.go

@@ -1,7 +1,5 @@
-// [ $A == 6 ] || errchk $G -e $D/$F.go
-
-// NOTE: This test is not run by 'run.go' and so not run by all.bash.
-// To run this test you must use the ./run shell script.
+// +build 386 arm
+// errorcheck
 
 // Copyright 2011 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -14,4 +12,4 @@ func main() {
 	var arr [1000200030]int   // ERROR "type .* too large"
 	arr_bkup := arr
 	_ = arr_bkup
-}
+}