crypto/win: Don't use disallowed APIs on UWP

crypto/win: Don't use disallowed APIs on UWP

CreateFiber and ConvertThreadToFiber are not allowed in Windows Store
(Universal Windows Platform) apps since they have been replaced by
their Ex variants which have a new dwFlags parameter.

This flag allows the fiber to do floating-point arithmetic in the
fiber on x86, which would silently cause corruption otherwise since
the floating-point state is not switched by default.

Switch to these "new" APIs which were added in Vista.

See: https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-createfiberex#parameters

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12400)