implement the native operating system version detection for FreeBSD

[andrewrk]

This is a follow-up task from #4550.

zig/lib/std/zig/system.zig

Lines 229 to 231 in 7617610

	.freebsd => {
	// TODO Detect native operating system version.
	},

zig/lib/std/target.zig

Lines 115 to 181 in 7617610

	/// The version ranges here represent the minimum OS version to be supported
	/// and the maximum OS version to be supported. The default values represent
	/// the range that the Zig Standard Library bases its abstractions on.
	///
	/// The minimum version of the range is the main setting to tweak for a target.
	/// Usually, the maximum target OS version will remain the default, which is
	/// the latest released version of the OS.
	///
	/// To test at compile time if the target is guaranteed to support a given OS feature,
	/// one should check that the minimum version of the range is greater than or equal to
	/// the version the feature was introduced in.
	///
	/// To test at compile time if the target certainly will not support a given OS feature,
	/// one should check that the maximum version of the range is less than the version the
	/// feature was introduced in.
	///
	/// If neither of these cases apply, a runtime check should be used to determine if the
	/// target supports a given OS feature.
	///
	/// Binaries built with a given maximum version will continue to function on newer operating system
	/// versions. However, such a binary may not take full advantage of the newer operating system APIs.
	pub const VersionRange = union {
	none: void,
	semver: Version.Range,
	linux: LinuxVersionRange,
	windows: WindowsVersion.Range,
	/// The default `VersionRange` represents the range that the Zig Standard Library
	/// bases its abstractions on.
	pub fn default(tag: Tag) VersionRange {
	switch (tag) {
	.freestanding,
	.ananas,
	.cloudabi,
	.dragonfly,
	.fuchsia,
	.kfreebsd,
	.lv2,
	.solaris,
	.haiku,
	.minix,
	.rtems,
	.nacl,
	.cnk,
	.aix,
	.cuda,
	.nvcl,
	.amdhsa,
	.ps4,
	.elfiamcu,
	.mesa3d,
	.contiki,
	.amdpal,
	.hermit,
	.hurd,
	.wasi,
	.emscripten,
	.uefi,
	.other,
	=> return .{ .none = {} },
	.freebsd => return .{
	.semver = Version.Range{
	.min = .{ .major = 12, .minor = 0 },
	.max = .{ .major = 12, .minor = 1 },
	},
	},