`_WIN64` is defined for all 64-bit targets, including Arm, so it doesn't
work for detecting `x86_64`. We can use `_M_X64` instead.
Signed-off-by: David Lechner <david@pybricks.com>
Since _M_IX86 is already being checked in the x86 case, it will never
be true in the xtensa case and can be removed.
Signed-off-by: David Lechner <david@pybricks.com>
