ada: Simplify handling of VxWorks-specific error codes for ENOENT

ada: Simplify handling of VxWorks-specific error codes for ENOENT

These error codes were defined on older versions of VxWorks (5, 6, 7
SR0540) and now they are either not defined or they fallback to
ENOENT. To handle these cases without using complex tests against
vxworks versions, leverage on __has_include and provide a fallback to
ENOENT if these error codes are not defined.

gcc/ada/

	* sysdep.c (S_dosFsLib_FILE_NOT_FOUND, S_nfsLib_NFSERR_NOENT):
	New macros, falback to ENOENT when not already defined.
	(__gnat_is_file_not_found_error): Use these new macros to remove
	tests against VxWorks flavors.