Add a helper function to resolve TLS variable addresses for FreeBSD.

Add a helper function to resolve TLS variable addresses for FreeBSD.

The fbsd_get_thread_local_address function accepts the base address of
a thread's DTV array and the base address of an object file's link map
and uses this to compute a TLS variable's address.  FreeBSD
architectures use an architecture-specific method to determine the
address of the DTV array pointer and call this helper function to
perform the rest of the address calculation.

	* fbsd-tdep.c (fbsd_pspace_data_handle): New variable.
	(struct fbsd_pspace_data): New type.
	(get_fbsd_pspace_data, fbsd_pspace_data_cleanup)
	(fbsd_read_integer_by_name, fbsd_fetch_rtld_offsets)
	(fbsd_get_tls_index, fbsd_get_thread_local_address): New function.
	(_initialize_fbsd_tdep): Initialize 'fbsd_pspace_data_handle'.
	* fbsd-tdep.c (fbsd_get_thread_local_address): New prototype.