freebsd: correctly define __FreeBSD_version to the first stable release

See: https://docs.freebsd.org/en/books/porters-handbook/versions

Closes #24819.

src/Compilation.zig

@@ -6921,7 +6921,7 @@ pub fn addCCArgs(
                         // We don't currently respect the minor and patch components. This wouldn't be particularly
                         // helpful because our abilists file only tracks major FreeBSD releases, so the link-time stub
                         // symbols would be inconsistent with header declarations.
-                        min_ver.major * 100_000,
+                        min_ver.major * 100_000 + 500,
                     }));
                 } else if (target.isNetBSDLibC()) {
                     const min_ver = target.os.version_range.semver.min;

src/libs/freebsd.zig

@@ -92,7 +92,7 @@ pub fn buildCrtFile(comp: *Compilation, crt_file: CrtFile, prog_node: std.Progre
             try acflags.appendSlice(&.{
                 "-DLOCORE",
                 // See `Compilation.addCCArgs`.
-                try std.fmt.allocPrint(arena, "-D__FreeBSD_version={d}", .{target.os.version_range.semver.min.major * 100_000}),
+                try std.fmt.allocPrint(arena, "-D__FreeBSD_version={d}", .{target.os.version_range.semver.min.major * 100_000 + 500}),
             });
 
             inline for (.{ &cflags, &acflags }) |flags| {