Tidy warn-execstack handling

Tidy warn-execstack handling

Make ld and bfd values consistent by swapping values 0 and 2 in
link_info.warn_execstack.  This has the benefit of making the value an
"extended" boolean, with 0 meaning no warning, 1 meaning warn, other
values a conditional warning.

Yes, this patch introduces fails on arm/aarch64.  Not a problem with
this patch but an arm/aarch64 before_parse problem.

bfd/
	* elflink.c (bfd_elf_size_dynamic_sections): Adjust
	warn_execstack test.
include/
	* bfdlink.h (warn_execstack): Swap 0 and 2 meaning.
ld/
	* configure.ac (DEFAULT_LD_WARN_EXECSTACK): Use values of 0,
	1, 2 consistent with link_info.warn_execstack.
	* ld.texi: Typo fixes.
	* lexsup.c (parse_args): Adjust setting of link_info.warn_execstack.
	(elf_static_list_options): Adjust help message conditions.
	* configure: Regenerate.