arch/xtensa: Fix xcc regression with ZSR

Turns out that xt-xcc will bail when faced with a real core-isa.h (it
wants you to rely on the builtins in the compiler).  Undefine __XCC__
to force it to actually parse and emit declarations for its own
header.

(Also adds a newline to the generated one-line C file to silence a warning)

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>