threads: enable access to `pthread_attr_get` functions (#357)

threads: enable access to `pthread_attr_get` functions (#357)

The pthreads API exposes functions for querying the attributes of a
thread. This change allows these functions to be compiled in the
`THREAD_MODEL=posix` build. Some functions are skipped (and documented);
they can be added if/when needed. This change is motivated by a
`libc-test` test that uses these functions.