Fix opam on OpenBSD

[kit-ty-kate]

Queued on top of #6216
Fixes #6215

For the backport to 2.3, do we want to backport the entire PR or just the OpenBSD fix?

[avsm]

Works great on my OpenBSD desktop:

$ uname -a
OpenBSD 7.5 GENERIC.MP#82 amd64

<><> Global opam variables ><><><><><><><><><><><><><><><><><><><><><><><><><><>
arch              x86_64            # Inferred from system
exe                                 # Suffix needed for executable filenames (Windows)
jobs              1                 # The number of parallel jobs set up in opam configuration
make              gmake             # The 'make' command to use
opam-version      2.4.0~alpha1~dev  # The currently running opam version
os                openbsd           # Inferred from system
os-distribution   openbsd           # Inferred from system
os-family         bsd               # Inferred from system
os-version        7.5               # Inferred from system
root              /home/avsm/.opam2 # The current opam root directory
switch            default           # The identifier of the current switch
sys-ocaml-arch    x86_64            # Target architecture of the OCaml compiler present on your system
sys-ocaml-cc      cc                # Host C Compiler type of the OCaml compiler present on your system
sys-ocaml-libc    libc              # Host C Runtime Library type of the OCaml compiler present on your system
sys-ocaml-system  openbsd           # Target system of the OCaml compiler present on your system
sys-ocaml-version 4.14.1            # OCaml version present on your system independently of opam, if any

One test fails:

File "tests/reftests/dune.inc", line 2082, characters 0-243:
2082 | (rule
2083 |   (targets opam-repo-143dd2a2f59f5befbf3cb90bb2667f911737fbf8)
2084 |   (action
2085 |    (progn
2086 |     (run mkdir -p %{targets})
2087 |     (run tar -C %{targets} -xzf %{dep:opam-archive-143dd2a2f59f5befbf3cb90bb2667f911737fbf8.tar.gz} --strip-components=1))))

but looks unrelated (that tar needs to be gtar due to the --strip-components)

[mndrix]

Thanks. This PR fixes #6215 for me. My opinion may not matter on the patch, but that looked good to me too. I like the switch to uname via C stubs.