windows: disable C++ ubsan regardless of ABI

The existing `-fno-sanitize=undefined` flag was gated on `abi == .msvc`
to avoid undefined `__ubsan_handle_*` references from simdutf/highway.
The same linker error reproduces on Windows GNU ABI for the same
reason: the Zig-bundled libraries don't provide a matching UBSan
runtime for these C/C++ objects in our build configurations.

Widen the condition to `os.tag == .windows` so both MSVC and GNU
Windows targets skip ubsan for these C++ deps.

src/build/SharedDeps.zig

@@ -836,10 +836,11 @@ pub fn addSimd(
             "-DSIMDUTF_NO_LIBCXX",
         );
 
-        // Disable ubsan for MSVC to avoid undefined references to
-        // __ubsan_handle_* symbols that require a runtime we don't link
-        // and bundle. Hopefully we can fix this one day since ubsan is nice!
-        if (target.result.abi == .msvc) try flags.appendSlice(b.allocator, &.{
+        // Disable ubsan for Windows C/C++ objects to avoid undefined
+        // __ubsan_handle_* references. The Zig libraries on Windows don't
+        // currently bundle a matching UBSan runtime for these objects in
+        // our build configurations (this affects both MSVC and GNU ABIs).
+        if (target.result.os.tag == .windows) try flags.appendSlice(b.allocator, &.{
             "-fno-sanitize=undefined",
             "-fno-sanitize-trap=undefined",
         });