code review 109390043: misc/nacl: add go_nacl_arm_exec, update README

misc/nacl/README

 Native Client
 =============
 
 This document outlines the basics of building and developing the Go runtime and programs in the Native Client (NaCl) environment.
 
-Go 1.3 supports two architectures
+Go 1.3 supports three architectures
 
  * nacl/386 which is standard 386.
  * nacl/amd64p32 which is a 64 bit architecture, where the address space is limited to a 4gb window.·
+ * nacl/arm which is 32-bit ARMv7A architecture with 1GB address space.
 
 For background it is recommended that you read http://golang.org/s/go13nacl.
 
 Prerequisites
 -------------
 
 Native Client programs are executed inside a sandbox, the NaCl runtime. This runtime must be installed before you can use NaCl programs.
 
 The NaCl distribution comes with an installer which ensures you have access to the latest version of the runtime. The version tracks the Chrome numbering scheme.
 
 # Download NaCl
 
 Download nacl_sdk.zip file from https://developers.google.com/native-client/dev/sdk/download, and unpack it. I chose /opt/nacl_sdk
 
 # Update
 
 The zip file contains a small skeleton that can be used to download the correct sdk. These are released every 6-8 weeks, in line with Chrome releases.
 ········
         % cd /opt/nacl_sdk
         % ./naclsdk update
 
-At this time pepper_33 is the stable version. If naclsdk downloads a later version, please adjust accordingly.
+At this time pepper_33 is the stable version. If naclsdk downloads a later version, please adjust accordingly. As of June 2014, only the canary sdk provides support for nacl/arm.
 
-The cmd/go helper scripts expect that the runtime loaders, sel_ldr_x86_{32,64} are in your path. I find it easiest to make a symlink from the NaCl distribution to my $GOPATH/bin directory.
+The cmd/go helper scripts expect that the runtime loaders, sel_ldr_{x86_{32,64},arm} and nacl_helper_bootstrap_arm are in your path. I find it easiest to make a symlink from the NaCl distribution to my $GOPATH/bin directory.
 
         % ln -nfs /opt/nacl_sdk/pepper_33/tools/sel_ldr_x86_32 $GOPATH/bin/sel_ldr_x86_32
         % ln -nfs /opt/nacl_sdk/pepper_33/tools/sel_ldr_x86_64 $GOPATH/bin/sel_ldr_x86_64
+        % ln -nfs /opt/nacl_sdk/pepper_canary/tools/sel_ldr_arm $GOPATH/bin/sel_ldr_arm
+        % ln -nfs /opt/nacl_sdk/pepper_canary/tools/nacl_helper_bootstrap_arm $GOPATH/bin/nacl_helper_bootstrap_arm # only required for NaCl/ARM.
 
 Support scripts
 ---------------
 
 Symlink the two scripts in this directory into your $PATH, just as you did with NaCl sdk above.
 
         % ln -nfs $GOROOT/go/misc/nacl/go_nacl_amd64p32_exec $GOPATH/bin/go_nacl_amd64p32_exec
         % ln -nfs $GOROOT/go/misc/nacl/go_nacl_386_exec $GOPATH/bin/go_nacl_386_exec
+        % ln -nfs $GOROOT/go/misc/nacl/go_nacl_arm_exec $GOPATH/bin/go_nacl_arm_exec
 
 Building and testing
 --------------------
 
 Building for NaCl is similar to cross compiling for other platforms. However, as it is not possible to ever build in a `native` NaCl environment, the cmd/go tool has been enhanced to allow the full build, all.bash, to be executed, rather than just the compile stage, make.bash.
 
 The cmd/go tool knows that if GOOS is set to `nacl` it should not try to execute any binaries itself. Instead it passes their execution to a support script which sets up a Native Client environment and invokes the NaCl sandbox.
 
 The script's name has a special format, go_$GOOS_$GOARCH_exec, so cmd/go can find it.
 
 In short, if the support scripts are in place, the cmd/go tool can be used as per normal.
 
 # Build and test Go for NaCl
 
 NaCl does not permit direct file system access. Instead, package syscall provides a simulated file system served by in-memory data. The script nacltest.bash is the NaCl equivalent of all.bash. It builds NaCl with an in-memory file system containing files needed for tests, and then it runs the tests.
 
         % cd go/src
         % env GOARCH=amd64p32 ./nacltest.bash