gcc-configure-target: Set native-system-header-dir for target gcc
Without this set, in the case host=target (which it does for on-target
gcc), the native header directory is set to the target sysroot with
no prefix. This means it would look for sdt.h on the build system
instead of in the target headers and this can lead to build failures
and is host contamination of the build.
The correct fix is to explicitly set the native header directory to
the correct location and then the headers get detected correctly.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
