Toolchain git-cloning streamlining with submodules

This commit adds binutils, gcc, glibc, linux-headers-for-nacl, and
newlib as submodules. This is useful so the user doesn't have to execute
so many steps for the build process. (Seven steps are reduced to one.)
Minor formatting improvements to README

Removed some trailing whitespace and reformatted the prereq package list
for better readability.