fix: Support Intel-based macOS

[db7]

Fixes some issues when running on Intel-based macOS:

1. the program cksum does not take the same arguments as in other BSDs, perhaps this is the general case also in newer macOS versions.
2. startnb.sh should not select cortex cpu, so I chose qemu64 instead

Tested on
Darwin 24.6.0 Darwin Kernel Version 24.6.0: Wed Oct 15 21:12:21 PDT 2025; root:xnu-11417.140.69.703.14~1/RELEASE_X86_64 x86_64

This sequence of commands work properly

make fetchimg
make SERVICE=base build
./startnb.sh -k kernels/netbsd-SMOL -iimages/base-amd64.img