Let CRT take care of the entry point for wWinMain if libc is linked

Fixes #7852

Before, the modified test would fail with:

```
error: lld-link: undefined symbol: wWinMain
    note: referenced by C:\Users\Ryan\Programming\Zig\zig-x86_64-windows-0.15.1\lib\libc\mingw\crt\crtexewin.c:66
    note:               libmingw32.lib(ucrtexewin.obj):(wmain)
```

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/std/start.zig b/lib/std/start.zig
index c22a36f24f..98f8e43b8a 100644
--- a/lib/std/start.zig
+++ b/lib/std/start.zig
@@ -61,6 +61,10 @@ comptime {
                 } else if (!@typeInfo(@TypeOf(root.main)).@"fn".calling_convention.eql(.c)) {
                     @export(&main, .{ .name = "main" });
                 }
+            } else if (native_os == .windows and builtin.link_libc and @hasDecl(root, "wWinMain")) {
+                if (!@typeInfo(@TypeOf(root.wWinMain)).@"fn".calling_convention.eql(.c)) {
+                    @export(&wWinMain, .{ .name = "wWinMain" });
+                }
             } else if (native_os == .windows) {
                 if (!@hasDecl(root, "WinMain") and !@hasDecl(root, "WinMainCRTStartup") and
                     !@hasDecl(root, "wWinMain") and !@hasDecl(root, "wWinMainCRTStartup"))
@@ -527,6 +531,10 @@ fn wWinMainCRTStartup() callconv(.withStackAlign(.c, 1)) noreturn {
     std.os.windows.ntdll.RtlExitUserProcess(@as(std.os.windows.UINT, @bitCast(result)));
 }
 
+fn wWinMain(hInstance: *anyopaque, hPrevInstance: ?*anyopaque, pCmdLine: [*:0]u16, nCmdShow: c_int) callconv(.c) c_int {
+    return root.wWinMain(@ptrCast(hInstance), @ptrCast(hPrevInstance), pCmdLine, @intCast(nCmdShow));
+}
+
 fn posixCallMainAndExit(argc_argv_ptr: [*]usize) callconv(.c) noreturn {
     // We're not ready to panic until thread local storage is initialized.
     @setRuntimeSafety(false);