cmake: toolchain/xt-clang: force assembler ID to GNU

xt-clang uses GNU Assembler (xt-as) based on binutils. However,
CMake doesn't recognize it when invoking through xt-clang. This
results in CMake going through all possible combinations of
command line arguments while invoking xt-clang to determine
assembler vendor. This multiple invocation of xt-clang
unnecessarily lengthens the CMake phase of build, especially
when xt-clang needs to obtain license information from remote
licensing servers. So here forces the assembler ID to be GNU to
speed things up a bit.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>