Use SecureZeroMemory in memwipe on Windows

In #7419 (moved), we use explicit_bzero/memset_s in memwipe when available.

Windows has SecureZeroMemory with the same semantics in Windows.h. ​https://msdn.microsoft.com/en-us/library/windows/desktop/aa366877.aspx

As long as we build our Windows binaries with headers that contain SecureZeroMemory, the code will work on any Windows version:

"This function is defined as the RtlSecureZeroMemory function (see WinBase.h). The implementation of RtlSecureZeroMemory is provided inline and can be used on any version of Windows (see WinNT.h.)"