Revert "x86: call gen_idt with $ZEPHYR_BASE too"

This reverts commit 37f4178f.

This change builds gen_idt in the zephyr project tree instead of
building it in outdir of the application. The build process should all
happen inside outdir and no binaries should be placed in the zephyr
tree.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>