Tigthen flag_pic processing in vxworks_override_options

Tigthen flag_pic processing in vxworks_override_options

This fixes spurious complaints about PIC mode not supported
from "gcc --help=...", on VxWorks without -mrtp. The spurious message
is emitted by vxworks_override_options, called with flag_pic == -1
when we're running for --help.

The change simply adjusts the check testing for "we're generating pic code"
to "flag_pic > 0" instead of just "flag_pic". We're not generating code at
all when reaching here with -1.

gcc/ChangeLog:

2020-10-14  Olivier Hainque  <hainque@adacore.com>

	* config/vxworks.c (vxworks_override_options): Guard pic checks with
	flag_pic > 0 instead of just flag_pic.