link.Lld: fix linking ELF shared libraries with -l on Windows

In Zig 0.17.0, we started lowering these to `-L/ -l:/path/to/lib.so` in
the LLD invocation, but that obviously doesn't work on Windows. Luckily,
LLD's, uh, curious approach to path concatenation means that we can just
pass `-L ""` instead of `-L/` and everything works.

Resolves: https://codeberg.org/ziglang/zig/issues/37257

src/link/Lld.zig

@@ -1163,7 +1163,10 @@ fn elfLink(lld: *Lld, arena: Allocator) !void {
             // object, we pass it on the CLI as "-l:/absolute/path/to/libfoo.so". This will treat
             // the given path not actually as an absolute path, but as relative to the library
             // search path, so the root directory must therefore be the only library search path.
-            try argv.append("-L/");
+            switch (builtin.os.tag) {
+                .windows => try argv.appendSlice(&.{ "-L", "" }),
+                else => try argv.append("-L/"),
+            }
 
             for (base.comp.link_inputs) |link_input| switch (link_input) {
                 .res => unreachable, // Windows-only

test/tests.zig

@@ -2484,6 +2484,51 @@ pub fn addCliTests(b: *std.Build) *Step {
         step.dependOn(&run_test.step);
     }
 
+    {
+        // Test (cross-)compiling for Linux using `-l` syntax. The build system only uses `-l` for
+        // system libraries (and therefore only for native builds), but it is possible and legal to
+        // use it when cross-compiling.
+        const tmp_path = b.tmpPath();
+
+        const build_libfoo = b.addSystemCommand(&.{
+            b.graph.zig_exe,
+            "build-lib",
+            "-target",
+            "x86_64-linux",
+            "-dynamic",
+        });
+        const libfoo = build_libfoo.addOutputFileArg2("libfoo.so", .{ .prefix = "-femit-bin=" });
+
+        build_libfoo.setCwd(tmp_path);
+        build_libfoo.setName("zig build-lib libfoo.so");
+        build_libfoo.expectStdOutEqual("");
+        build_libfoo.expectStdErrEqual("");
+
+        const build_libbar = b.addSystemCommand(&.{
+            b.graph.zig_exe,
+            "build-lib",
+            "-target",
+            "x86_64-linux",
+            "-dynamic",
+        });
+        build_libbar.addDirectoryArg2(libfoo.dirname(), .{ .prefix = "-L" });
+        build_libbar.addArg("-lfoo");
+        const libbar = build_libbar.addOutputFileArg2("libbar.so", .{ .prefix = "-femit-bin=" });
+
+        build_libbar.setCwd(tmp_path);
+        build_libbar.setName("zig build-lib libbar.so");
+        build_libbar.expectStdOutEqual("");
+        build_libbar.expectStdErrEqual("");
+
+        // We mainly just want to make sure the build commands complete successfully, but we may as
+        // well double-check that the output is a valid ELF file.
+        const check_libbar = b.addCheckFile(libbar, .{
+            .expected_matches = &.{"\x7fELF"},
+        });
+
+        step.dependOn(&check_libbar.step);
+    }
+
     return step;
 }