Define xtensa-specific info_is_fdpic and fill in FDPIC-specific
registers in the xtensa version of init_thread.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20230205061230.544451-1-jcmvbkbc@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>