[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HowToCrossCompile was outdated and generating too much traffic on the mailing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