[docs] Change CrossCompilation guidde to reflect changes in Clang/GCC
HowToCrossCompile was outdated and generating too much traffic on the mailing
list with similar queries. This change helps offset most of the problems that
were reported recently including:
- Removing the -ccc-gcc-name, adding --sysroot
- Making references to Debian's multiarch for target libraries
- Expanding -DCMAKE_CXX_FLAGS for both GCC and Clang
- Some formatting and clarifications in the text