cmake: force assembler ID to be GNU for GCC-based XCC

GCC-based XCC uses GNU Assembler (xt-as). However, CMake doesn't
recognize it when invoking through xt-xcc. This results in CMake
going through all possible combinations of command line arguments
while invoking xt-xcc to determine assembler vendor. This multiple
invocation of xt-xcc unnecessarily lengthens the CMake phase of
build, especially when XCC 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>