Make core_unix.v0.15.x unavailable on FreeBSD 14+

Similar to #26013 this makes core_unix unavailable on FreeBSD. The code
seems to be the same. In older versions the code is slightly different
so it might work.

Failures look like this:

ld: error: undefined symbol: core_linux_timerfd_create
>>> referenced by linux_ext.o:(.text+0x464D) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a
>>> referenced by linux_ext.o:(.data+0x2E38) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a

ld: error: undefined symbol: core_linux_timerfd_settime
>>> referenced by linux_ext.o:(.text+0x48B1) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a
>>> referenced by linux_ext.o:(.text+0x4B91) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a
>>> referenced by linux_ext.o:(.data+0x2E30) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a

ld: error: undefined symbol: core_linux_timerfd_gettime
>>> referenced by linux_ext.o:(.text+0x4AEA) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a
>>> referenced by linux_ext.o:(.data+0x2E28) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a

ld: error: undefined symbol: core_linux_timerfd_CLOCK_REALTIME
>>> referenced by linux_ext.o:(.text+0x7B6A) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a
>>> referenced by linux_ext.o:(.data+0x2E58) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a

ld: error: undefined symbol: core_linux_timerfd_CLOCK_MONOTONIC
>>> referenced by linux_ext.o:(.text+0x7B84) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a
>>> referenced by linux_ext.o:(.data+0x2E50) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a

ld: error: undefined symbol: core_linux_timerfd_TFD_NONBLOCK
>>> referenced by linux_ext.o:(.text+0x7C36) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a
>>> referenced by linux_ext.o:(.data+0x2E48) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a

ld: error: undefined symbol: core_linux_timerfd_TFD_CLOEXEC
>>> referenced by linux_ext.o:(.text+0x7C4F) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a
>>> referenced by linux_ext.o:(.data+0x2E40) in archive /home/opam/.opam/4.14.2/lib/core_unix/linux_ext/linux_ext.a

Author: Leonidas-from-XIV

packages/core_unix/core_unix.v0.15.0/opam

@@ -29,6 +29,7 @@ description: "
 Unix-specific extensions to some of the modules defined in [core] and [core_kernel].
 "
 depexts: ["linux-headers"] {os-family = "alpine"}
+available: [ !(os = "freebsd" & os-version >= "14") ]
 url {
 src: "https://ocaml.janestreet.com/ocaml-core/v0.15/files/core_unix-v0.15.0.tar.gz"
 checksum: "sha256=0af9d2c0d2029a80858c730171e0bd70a1981b3e7021f8c31cd8dc54925da02d"

packages/core_unix/core_unix.v0.15.1/opam

@@ -29,6 +29,7 @@ description: "
 Unix-specific extensions to some of the modules defined in [core] and [core_kernel].
 "
 depexts: ["linux-headers"] {os-family = "alpine"}
+available: [ !(os = "freebsd" & os-version >= "14") ]
 url {
 src: "https://github.com/ocaml/opam-source-archives/raw/main/core_unix-v0.15.1.tar.gz"
 checksum: "sha256=b467c7c64616d5cc88f42f9660b447247e7a848148336e0ad0de6bd35edce9b3"

packages/core_unix/core_unix.v0.15.2/opam

@@ -29,6 +29,7 @@ description: "
 Unix-specific extensions to some of the modules defined in [core] and [core_kernel].
 "
 depexts: ["linux-headers"] {os-family = "alpine"}
+available: [ !(os = "freebsd" & os-version >= "14") ]
 url {
 src: "https://github.com/janestreet/core_unix/archive/refs/tags/v0.15.2.tar.gz"
 checksum: "sha256=486d0e954603960fa081b3fd23e3cc3e50ac0892544acd35f9c2919c4bf5f67b"