tests: Enable qemu_xtensa logging tests

Enables logging testing for qemu_xtensa along with the fixes needed to
correctly run the tests.

Several tests in log_api rely on being able to calculate the size of
the message in the mpsc_pbuf to know when an overflow occurs. The
size being calculated assumed a ROUND_UP size alignment to sizeof(long
long). On xtensa this is actually 16 bytes as defined by
CBPRINTF_PACKAGE_ALIGNMENT.

Changes the size calculation for SIMPLE_MSG_LEN to account for this.

The second issue was a misaligned package that would cause reading
from the incorrect memory when going to format the package with cbpprintf.

Fixes the padding used to align the package in cbprintf.h and adds a build
assert ensuring correct alignment when building against xtensa in both
cbprintf.h and log_msg.h.

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