Remove current_target_sections macro

Remove current_target_sections macro

This removes the current_target_sections macro, replacing it with uses
of the appropriate member from current_program_space.

gdb/ChangeLog
2020-10-29  Tom Tromey  <tom@tromey.com>

	* progspace.h (current_target_sections): Remove macro.
	* solib-svr4.c (scan_dyntag): Update.
	* solib-dsbt.c (scan_dyntag): Update.
	* exec.c (exec_target::close): Update.
	(add_target_sections, add_target_sections_of_objfile)
	(remove_target_sections, exec_target::get_section_table)
	(exec_target::files_info, set_section_command)
	(exec_set_section_address, exec_target::has_memory)
	(exec_target::has_memory): Update.