stubs: fix Windows x86 TLS callback CC violation

Windows x86 requires that the TLS destructor callback uses the "stdcall"
calling convention.  Provide a shim which will adjust the calling
convention and call back into the swift portion of the code code with
the expected calling convention.

stdlib/public/core/ThreadLocalStorage.swift

@@ -101,6 +101,7 @@ internal struct _ThreadLocalStorage {
 
 // Destructor to register with pthreads. Responsible for deallocating any memory
 // owned.
+@_silgen_name("_swift_stdlib_destroyTLS")
 internal func _destroyTLS(_ ptr: UnsafeMutableRawPointer?) {
   _sanityCheck(ptr != nil,
     "_destroyTLS was called, but with nil...")