time: unconditionally use RegLoadMUIString on Windows

time: unconditionally use RegLoadMUIString on Windows

RegLoadMUIString is supported on Windows Vista and later, so this
should be safe to do unconditionally.

While here, also unconditionally use GetDynamicTimeZoneInformation
in syscall/windows/registry tests, since it's also supported on Windows
Vista and later.

Change-Id: Ifa32a81f4727d4810f2b9ccc814aa77a43513e59
Reviewed-on: https://go-review.googlesource.com/c/go/+/527595
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>