soc/intel_adsp: Add workarounds for building bootloader with xcc

The bootloader code on this SOC has its own cmake rules, which means
it doesn't get the Zephyr-specific magic from the toolchain layer and
the code needs to handle fixups manually.  (Specifically: we have a
xcc_missing_defs.h header to provide gcc symbols that xcc doesn't
have, and assembly needs to be built with _ASMLANGUAGE so headers
don't include C syntax.)

Long term the right solution here is to build the bootloader as part
of the Zephyr binary.

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