test: disable SILGen test

This does not make sense on PE/COFF targets where the `_weakLinked`
attribute is unsupported.

Author: compnerd

test/SILGen/weak_linked_attribute.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-emit-silgen %s | %FileCheck %s
+// UNSUPPORTED: OS=windows-msvc
 
 // CHECK-LABEL: sil [_weakLinked] [ossa] @$s21weak_linked_attribute0A8FunctionyyF : $@convention(thin) () -> ()
 @_weakLinked public func weakFunction() {}