Change add_target_sections to method on program_space

Change add_target_sections to method on program_space

This changes add_target_sections to be a method on program_space.
Like the earlier change to remove_target_sections, this makes sense
because this function is manipulating data that is stored on the
program space.

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

	* solib.c (solib_map_sections): Update.
	* exec.c (program_space::add_target_sections): Now a method.
	(exec_file_attach): Update.
	* exec.h (add_target_sections): Don't declare.
	* progspace.h (struct program_space) <add_target_sections>:
	Declare.