Rework space/subspace handling in PA code to fully support

	Rework space/subspace handling in PA code to fully support
	SOM spaces/subspaces.
	* config/tc-hppa.c (USE_ALIASES): New object-format dependent define
	to control the use of space/subspace name aliases.
	(update_subspace): Accept space chain entry for containing space
	as a new parameter.  All callers changed.
	(pa_get_label): Use current_space rather than pa_segment_to_space.
	(pa_define_label): Likewise.
	(pa_undefine_label): Likewise.
	(md_begin): Change into the (possibly modified) text_section.
	(pa_parse_space_stmt): Create a new segment/space if create_flag
	is true, and the space name is not one of the two predefined spaces.
	(pa_subspace): Use current_space rather than a lookup via
	pa_segment_to_space.  Reset BFD section flags as required by
	the .subspace directive.  Likewise for the section alignment.
	Pass the current space to update_subspace and create_new_subspace.
	(pa_spaces_begin): Only use space/subspace aliases if USE_ALIASES
	is true.  When not using aliases, create a BFD section for each
	subspace encountered.  When not using aliases replace the default
	text, data, and bss segments with new ones.
	(create_new_subspace): When not using aliases each subspace has a
	section/segment and subsegments are not needed, so set the subsegment
	to zero.