bfd: stop over-allocating aux entries for COFF section symbols

coff_new_section_hook allocated ten combined_entry_type slots for every
section symbol, behind a comment conceding that the ten was a guess and
should not be a constant.  Nothing in BFD sets n_numaux above 1 on a
section symbol, and nothing indexes the array past native[1], so eight
of the ten were never touched on any target.

This runs once per input section, so it significantly reduces peak
memory usage.

bfd/
	* coffcode.h (coff_new_section_hook): Allocate one syment plus
	one aux entry rather than ten.

Signed-off-by: Oleg Tolmatcev <oleg.tolmatcev@gmail.com>