xtools: get rid of warnings about wrong path

Eliminate following errors:

make[2]: /home/nashif/Work/sdk/xtools/outdir/x-tools//bin/-gcc: Command
not found
make[2]: /home/nashif/Work/sdk/xtools/outdir/x-tools//bin/-gcc: Command
not found
make[2]: /home/nashif/Work/sdk/xtools/outdir/x-tools//bin/-gcc: Command
not found

Also, usage of CROSS_COMPILE with a predefined toolchain is not required and
complicates things, just call CROSS_COMPILE with the full path if you
want to build with a toolchain not supported with Zephyr.

Change-Id: I93ec4ff2e04d22cee82c8e4b74b652927572b30a
Signed-off-by: Anas Nashif <anas.nashif@intel.com>