xtensa: provide XCC compiler support for Xtensa

This patchset provides Xtensa's xcc compiler support for Xtensa
projects in Cmake. This requires the below environment variables
to be defined aptly. The appropriate xcc license information also
need to be supplied.

ZEPHYR_GCC_VARIANT=xcc
TOOLCHAIN_VER=RF-2015.3-linux
XTENSA_CORE=cavs21_LX6HiFi3_RF3_WB16
XTENSA_SYSTEM=/opt/xtensa/XtDevTools/install/tools/
		RF-2015.3-linux/XtensaTools/config/
XTENSA_BUILD_PATHS=/opt/xtensa/XtDevTools/install/builds/

Change-Id: Ib3c10e8095439b0e32276ff37c00eca8420773ec
Signed-off-by: Rajavardhan Gundi <rajavardhan.gundi@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>