arm_kernel_bothdr.awk: Update to latest ota

The latest ota is the first one in FreeBSD that treats 0 + "0xf" as
being '0' instead of '15'. Don't use this old trick anymore to convert
from hexidecimal to a number. Write a function to do that instead. This
fixes kernel.bin building on arm*. awk on 14 doesn't need this, but to
build FreeBSD stable/14's kernel.bin on 15 we'll need it, so fast MFC.

MFC After: 3 days
Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44801