gdb-cross used to be specific to the tune flags, but isn't
anymore. Therefore it is enough to use TARGET_SYS instead of
TUNE_PKGARCH to create a unique path.
Fixes a sstate signature difference that was found via
yocto-compat-layer.py's test_machine_signatures check. In practice it
probably showed up as unnecessarily rebuilding gdb-cross when
switching between machines like intel-corei7-64 and qemux86-64.
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
