[Ada] Handle version 2 of Windows unwinding information structures

[Ada] Handle version 2 of Windows unwinding information structures

2018-05-24  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* raise-gcc.c (__gnat_SEH_error_handler): Remove prototype.
	(__gnat_personality_seh0): Adjust and beef up comments, and
	fix formatting throughout.
	(__gnat_adjust_context): Deal minimally with version 2.
	* seh_init.c (__gnat_map_SEH): Fix formatting.
	(_gnat_SEH_error_handler): Adjust comments.
	(__gnat_install_SEH_handler): Fix formatting.

From-SVN: r260659