Use xgo in build.sh for easier cross compilation

[ignassew]

xgo: https://github.com/techknowlogick/xgo

I tried to write a cross-platform script that uses only the go compiler, but it's very (you have to extract mac-specific SDKs from Xcode, build and compile a toolchain for mac).

xgo seemed to be the easiest way for cross compilation. it uses docker on the backend, It should work on any platform

I tested compilation and example_python on x86_64 linux, can't say for other platforms.

[bogdanfinn]

@ignassew thanks for contribution. i will take a look and also make some tests before merging it :-)