intel_adsp: bbzero/bmemcpy with picolibc fix

When building with picolibc and gcc, the loops to do zeroing/copying
get replaced by gcc with calls to memset/memcpy. This fails this early in
the boot process and results in an illegal instruction exception.

Marking the variables being manipulated in the calls as volatile prevents
the compiler from optimizing the loops (replacing them with memset/memcpy).

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>