xcc/cmake: fix error message when TOOLCHAIN_VER is undefined

The exact set of environment variables required by xt-xcc and xt-clang
is a bit of a dark magic and error-prone. TOOLCHAIN_VER is definitely
one of them and the error message was puzzling when TOOLCHAIN_VER was
undefined or not exported.

- Add TOOLCHAIN_VER to the list of variables in the error message

- Replace the vague "toolchain not found" in the error message with the
  more useful "XCC/install/tools//XtensaTools/bin/xt-xcc not found" where
  the double slash and comparison with the filesystem clearly point at
  where TOOLCHAIN_VER is missing.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>