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HowToCrossCompile was outdated and generating too much traffic on the mailing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

llvm-svn: 269054