cmd: replace bio.Buf with bio.Reader and bio.Writer

Replace the bidirectional bio.Buf type with a pair of unidirectional
buffered seekable Reader and Writers.

Change-Id: I86664a06f93c94595dc67c2cbd21356feb6680ef
Reviewed-on: https://go-review.googlesource.com/21720
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/cmd/compile/internal/gc/go.go b/src/cmd/compile/internal/gc/go.go
index cd9db38..ec7e219 100644
--- a/src/cmd/compile/internal/gc/go.go
+++ b/src/cmd/compile/internal/gc/go.go
@@ -133,7 +133,7 @@
 
 var outfile string
 
-var bout *bio.Buf
+var bout *bio.Writer
 
 var nerrors int
 
@@ -288,7 +288,7 @@
 
 var writearchive int
 
-var bstdout *bio.Buf
+var bstdout *bio.Writer
 
 var Nacl bool