wic: remove systemd-boot for x32

wic: remove systemd-boot for x32

Currently systemd-boot actually incorporates libgcc, since the
systemd-boot needs to be built with 64bit instructions it can not
use the x32 based libgcc.

Use the new override to ensure it gets overriden, linux-gnux32 could
not be used because x86-64 has higher priority.

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>