sigrok-cross-mingw: move libusb1 manipulation out of "MXE fixes"

sigrok-cross-mingw: move libusb1 manipulation out of "MXE fixes"

Factor the libusb1 related tweaking out of the "MXE fixes" patch file.
It is assumed that binutils and glib issues are rather different from
the libusb manipulation. A separate libusb patch also simplifies its
maintenance.

Add comments discussing how the "event abstraction" branch reference
may have become obsolete when libusb1 1.0.24 was released. Which only
leaves RAW_IO as an essential motivation for a modified version (which
is also being submitted towards mainline libusb).