soc/intel_adsp: Fix _ASMLANGUAGE declaration for boot_entry.S

The cAVS bootloader code (for... somewhat silly reasons) doesn't build
via the same paths as regular Zephyr object files, so it wasn't
getting the _ASMLANGUAGE define.  That meant that Zephyr headers
defining BIT() were using syntax incompatible with some assemblers
(specifically the Cadence xcc assembly; current gas versions were
fine).

Not 100% sure this is the best spot to put this, but the root fix is
to get the bootloader building into the same link as the rest of
Zephyr anyway.

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