add cross-compiling

[philippe44]

The main purpose of this PR is to add cross-compiling to your library

• build.sh is a script example
• when cross-compiling, test is disabled and custom_tools are build with native compiler
• endianness is set automatically by compiler
• big endian mode was not compiling, I think I've fixed it properly
• the targets/ directory is something I generally provide on all libs I use to have a set of pre-built libraries (can be ignored)