When building depends on FreeBSD/OpenBSD `aarch64`, the host compilers
default to `default_host_{CC,CXX}`, which resolves to `gcc`/`g++`. This
is incorrect on these systems, where Clang is the default system
compiler.
On FreeBSD, the `shasum` utility is provided by the `perl5` port, which
is not part of the base system and must be installed separately.
Note that this requirement is currently not documented in
`depends/README.md`.
This change switches to using the `sha256sum` utility, which is included
in the base system.
