x86: call gen_idt with $ZEPHYR_BASE too

When calling scripts/gen_idt, if we don't have $ZEPHYR_BASE/scripts in
the path, it will fail, so we can call it with its full path to avoid
such need.

Change-Id: I47b340c9f3204ad8740c29e663e12082208bb13b
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>