[test] Add UNSUPPORTED: system-windows to linux-ld.c

We should have a test verifying / \ for Windows but have such a long
test specifically for Linux cross compilation suffer from Windows \
is too troublesome.

clang/test/Driver/linux-ld.c

@@ -1,3 +1,4 @@
+// UNSUPPORTED: system-windows
 // General tests that ld invocations on Linux targets sane. Note that we use
 // sysroot to make these tests independent of the host system.
 //