sys/util.h: Make IS_ENABLED work for linker scripts

Z_IS_ENABLED2 uses true and false, however when we preprocess the linker
scripts we invoke the compiler with `-x assembler-with-cpp` so
_ASMLANGUAGE ends up being defined and thus stdbool.h wouldn't convert
'true' and 'false' to '1' and '0'.

Move the include of stdbool.h outside the #ifndef _ASMLANGUAGE check so
that 'true' and 'false' get converted correctly.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>