misc/ios: add README and wrapper script to ease cross-compiling for darwin/arm

Fixes #837.

Change-Id: I2d601504addbd220b304d32d587144d2a702f753
Reviewed-on: https://go-review.googlesource.com/2127
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

misc/ios/README

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+Go on iOS
+=========
+
+To build a cross compiling toolchain for iOS on OS X, first modify clangwrap.sh
+in misc/ios to match your setup. And then run:
+
+	GOARM=7 CGO_ENABLED=1 GOARCH=arm CC_FOR_TARGET=`pwd`/../misc/ios/clangwrap.sh \
+	CXX_FOR_TARGET=`pwd`/../misc/ios/clangwrap.sh ./make.bash
+
+To build a program, use the normal go build command:
+
+	CGO_ENABLED=1 GOARCH=arm go build import/path
+
+To run a program on an iDevice, first make sure you have a valid developer
+certificate and have setup your iDevice properly to run apps signed by your
+developer certificate. Then install https://github.com/phonegap/ios-deploy.
+At a first step, you can try building the famous hello world program to run
+on your test device.
+(The needed files are provided at https://github.com/minux/go-ios-examples.)
+
+	# assume your program binary is helloworld.go, build it into the
+	# example hello.app bundle.
+	CGO_ENABLED=1 GOARCH=arm go build -o hello.app/hello helloworld.go
+	# sign the executable using your developer certificate
+	codesign -f -s "iPhone Developer" --entitlements hello.app/Entitlements.plist hello.app/hello
+	# run the program inside lldb on iDevice, run `ios-deploy` for more
+	# command options
+	ios-deploy --debug --uninstall --bundle hello.app
+	# Depending on your ios-deploy version, you might need to enter "run"
+	# into lldb to run your program, and its output will be shown by lldb.
+
+Notes:
+ - A dummy hello.app bundle is provided in this directory to help you get started.
+ - Running the program on an iDevice requires code sign and thus external linking,
+   if your program uses cgo, then it will automatically use external linking.
+   However, if your program does not use cgo, please make sure to add
+	import _ "runtime/cgo"
+   so that external linking will be used.
+
+Known issues
+============
+ - crypto/x509 won't build, I don't yet know how to get system root on iOS.
+ - Because I still want to be able to do native build, CGO_ENABLED=1 is not the
+   default, yet.

misc/ios/clangwrap.sh

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+#!/bin/sh
+# This uses the latest available iOS SDK, which is recommended.
+# To select a specific SDK, run 'xcodebuild -showsdks'
+# to see the available SDKs and replace iphoneos with one of them.
+SDK=iphoneos
+SDK_PATH=`xcrun --sdk $SDK --show-sdk-path`
+export IPHONEOS_DEPLOYMENT_TARGET=5.1
+# cmd/cgo doesn't support llvm-gcc-4.2, so we have to use clang.
+CLANG=`xcrun --sdk $SDK --find clang`
+exec $CLANG -arch armv7 -isysroot $SDK_PATH "$@"