doc: when configuring gold for gccgo, use --enable-gold=default

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5030057

doc/gccgo_install.html

@@ -58,7 +58,7 @@ use a small discontiguous stack for goroutines.  This permits programs
 to run many more goroutines, since each goroutine can use a relatively
 small stack.  Doing this requires using a development version of
 the <code>gold</code> linker.  The easiest way to do this is to build
-the GNU binutils, using <code>--enable-gold</code> when you run
+the GNU binutils, using <code>--enable-gold=default</code> when you run
 the <code>configure</code> script, and to
 use <code>--with-ld=GOLD_BINARY</code> when you
 configure <code>gccgo</code>.  A typical sequence would look like
@@ -72,7 +72,7 @@ cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co binutils
 mkdir binutils-objdir
 cd binutils-objdir
-../src/configure --enable-gold --prefix=/opt/gold
+../src/configure --enable-gold=default --prefix=/opt/gold
 make
 make install
 </pre>