Fix logic for -fno-stack-protector arg in xenomai

FIXME Getting architecture from the build host will break e.g. 32-bit
builds on 64-bit systems.  Need an expert to figure out host/target
arch like --host=i686-pc-linux-gnu even when
--build=x86_64-pc-linux-gnu

One environment affected is package builds
CFLAG fixes:  match pre-universal build and more cleanups

xenomai-kernel:  add missing -g back to kbuild

rtapi_compat.c:  universal sources don't need flavor-specific RTFLAGS;
  removing the copy added in rtapi/Submakefile (but leaving the second
  copy from hal/Submakefile)

rtapi/flavor.c, rtapi_kdetect.c: universal sources don't need
  flavor-specific RTFLAGS