Simplify add_target_sections_of_objfile

Now that target_section_table uses std::vector,
add_target_sections_of_objfile does not need to loop twice.  This
patch simplifies this code to have just a single loop.  Also, the
passed-in objfile can never be NULL, so this changes this function to
assert that.

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

	* exec.c (add_target_sections_of_objfile): Simplify.