This allows building the libc & libm pages in parallel, and drops
the duplication in the subdirs with the chew/chapter settings.
The unused rules in Makefile.shared are left in place to minimize
noise in the change.
This doesn't migrate all the docs, just the libc's manual (pdf/info).
This is to show the basic form of migrating the chew files.
For subdirs that didn't have any docs, I've stripped their settings
for clarity. If someone wanted to suddenly add docs, they can add
the corresponding Makefile.inc files easily.