nacltest.bash, misc/nacl/README: update NaCl docs.

LGTM=rsc
R=dave, rsc
CC=golang-codereviews
https://golang.org/cl/105030043

misc/nacl/README

@@ -54,12 +54,10 @@ The script's name has a special format, go_$GOOS_$GOARCH_exec, so cmd/go can fin
 
 In short, if the support scripts are in place, the cmd/go tool can be used as per normal.
 
-# Build the Go toolchain.
+# Build and test Go for NaCl
 
-	% cd go/src
-	% env GOOS=nacl GOARCH=amd64p32 ./make.bash
-
-# Test the Go toolchain.
+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 GOOS=nacl GOARCH=amd64p32 ./run.bash
+	% env GOARCH=amd64p32 ./nacltest.bash
+

src/nacltest.bash

@@ -7,7 +7,8 @@
 # Builds a test file system and embeds it into package syscall
 # in every generated binary.
 #
-# Assumes that sel_ldr binaries are in $PATH; see ../misc/nacl/README.
+# Assumes that sel_ldr binaries and go_nacl_$GOARCH_exec scripts are in $PATH;
+# see ../misc/nacl/README.
 
 set -e
 ulimit -c 0
@@ -32,6 +33,11 @@ amd64p32)
 	exit 1
 esac
 
+if ! which go_nacl_${naclGOARCH}_exec >/dev/null; then
+	echo "cannot find go_nacl_${naclGOARCH}_exec, see ../misc/nacl/README." 1>&2
+	exit 1
+fi
+
 # Run host build to get toolchain for running zip generator.
 unset GOOS GOARCH
 if [ ! -f make.bash ]; then