Update README.chromium for gold linker.

README.chromium

 Name: gold
 URL: http://www.gnu.org/software/binutils/
-Version: 2.21.1
+Version: 2.23.1
 License: GPL v2
 Security Critical: no
 
 Description:
-This directory contains a binary of the gold linker from GNU binutils.
-It was built from binutils-2.21.1 on 64-bit Ubuntu Lucid as follows:
+This directory contains i386 and amd64 binaries of the gold linker from GNU
+binutils.  They were built from binutils-2.23.1 on Ubuntu Lucid as follows:
 
-1) Fetch tarball: http://ftp.gnu.org/gnu/binutils/binutils-2.21.1.tar.bz2
+1) Fetch tarball: http://ftp.gnu.org/gnu/binutils/binutils-2.23.1.tar.bz2
 2) Unpack, then
     ./configure --enable-gold=default --enable-threads
-3) make maybe-all-gold; strip gold/ld-new
-4) mv gold/ld-new path/to/this/dir/gold64
+3) make maybe-all-gold && strip gold/ld-new
+4) mv gold/ld-new path/to/this/dir/gold64 (or gold32)
+
+To build on a non-Lucid (e.g. Precise) machine, or to build the 32-bit binary
+on a 64-bit machine, a chroot can be used by running the following commands as
+root (swap i386 for amd64 as needed):
+
+  debootstrap --arch=i386 --include=build-essential,flex,bison \
+    lucid i386-lucid
+  chroot i386-lucid
+
+Then follow steps 1-4 above.
 
 (ld.bfd is a forwarding script that is in place because the NaCl build
 depends on there being a script with that name in the same directory as
 ld.)