cmake: xcc/xt-clang: fix for proper system include paths

Although xt-clang is based on clang, for some reason, it still
lists xcc system include path as the first search path (e.g.
for stddef.h), and the clang system include path as last. This
creates a big issue when the code starts to use any standards
past C89 (since xcc is based on GCC 4.2). We can use compiler
property nostdin_include to add -isystem to compiler options.
However, some modules (e.g. picolibcs) somehow ignore this.
So we also need to forcibly do add_compile_options() to make
sure the clang system include path is placed before the xcc
system include path.

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