fixed installation procedures
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.
Adds submodule for gdb
Revised and simplified build instructions
Revisions/cleanup to env var section of README
Updated gcc submodule
Pegged glibc submodule to an older working version
Fixed the version of the gcc submodule