posix: deprecate PTHREAD_RWLOCK for POSIX_READER_WRITER_LOCKS

This change deprecates CONFIG_PTHREAD_RWLOCK in favour of
CONFIG_POSIX_READER_WRITER_LOCKS, which maps directly to the
name of the standard POSIX Option.

Annoyingly, the POSIX_RW_LOCKS Option Group is inconsistently
named. However, it is more convenient for us to use the Option
name since it is also the format used by the sysconf() variable
(_SC_READER_WRITER_LOCKS).

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>