Introduce target-section.h

This introduces a new target-section.h file.  This makes some of the
later patches in this series a bit cleaner, because new includes of
target.h won't be required.  Also I think it's better to have small
header files for each separate data structure.

gdb/ChangeLog
2020-10-12  Tom Tromey  <tom@tromey.com>

	* target.h (struct target_section, struct target_section_table):
	Move to target-section.h.
	* target-section.h: New file.