This patch adds entries for the c++tools, gotools, libbacktrace,
libcc1, libcody, liboffloadmic, and libsanitizer directories into the
list of toplevel source directories in sourcebuild.texi. I also
removed the entry for boehm-gc (which is no longer in-tree), and fixed
the alphabetization for libquadmath while I was at it. Any style nits
I need to fix before committing (with a proper ChangeLog entry)?