Add cross-compiling toolchain files for Android and Raspberry Pi.

[Xreki]

• 增加了交叉编译Android、Raspberry Pi的cmake toolchain文件，自动地设置编译器、编译参数。
• 自动地编译host protobuf和target protobuf，OpenBLAS。

相关PR：#1698， #1835

[hedaoyuan]

I can use raspberry_pi.cmake compiler raspberry pi version. But if use the make j command, the compilation process may be interrupted due to the error.

[hedaoyuan]

这个不需要判断吧，include(raspberry_pi.cmake)前已经判断了。

[Xreki]

每个toolchain file最开始都做了双重判断。留着保险一些吧。

[Xreki]

@hedaoyuan Thank you for your try. I have reproduced the compiling error. It is because of the misuse of LIST(APPEND variable ...). Variables can not be modified in function using LIST(APPEND variable ... PARENT_SCOPE) like SET(variable ... PARENT_SCOPE). Thus the dependencies of protobuf_host and protobuf actually were not appended to external_project_dependencies. It is fixed in the latest commit and was tested using make -j12.

[gangliao]

用这个commit���什么原因呢 支持android?

[Xreki]

OpenBLAS为支持softfp新开的一个分支arm_soft_fp_abi，主分支只支持hardfp，Android上运行有些问题。

[gangliao]

明白了

[Xreki]

我加个注释说明一下吧~

[gangliao]

👍

[gangliao]

LGTM

[Xreki]

@hedaoyuan 建议写一个如何编写交叉编译toolchain文件的设计文档，该文档将和如何交叉编译ARM版Paddle的使用文档一起，在下一个PR中提交。