Change subdir traversal to primarily cache information about what is available in subdirectories.

 - Rest of makefiles will move to using the information after it has been computed, instead of during subdir traversal.

Also, add 'make info-functions' target, which prints information on all the functions available in compiler-rt.

Also, add 'make help-devel' for listing help on targets intended for compiler-rt developers or direct users.

llvm-svn: 93715