[Ada] Vxworks7r2 SR0610 coalesced some macro values

[Ada] Vxworks7r2 SR0610 coalesced some macro values

SR0600 and SR0610 cannot be differentiated by macro testing (arguably an
oversight in header file version.h) so: The case statement testing for
"file not found" is reformulated into an if/else series of statements to
avoid a problem where two cases have identical values in SR0610, but
different values in SR0600.

2019-07-10  Doug Rupp  <rupp@adacore.com>

gcc/ada/

	* sysdep.c (__gnat_is_file_not_found_error): Reformulate to also
	work for vxworks7r2 SR0610.

From-SVN: r273333