cmake: use sdk-ng built toolchain for x86_64

This adds the necessary bits to utilize the x86_64 toolchain
built by sdk-ng for x86_64 when toolchain variant is either
zephyr or xtools. This allows decoupling the builds from
the host toolchain.

Newlib is also available with this toolchain so remove
the Kconfig restriction on CONFIG_NEWLIB_LIBC.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>