ld: Set default subsystem for arm-pe to IMAGE_SUBSYSTEM_WINDOWS_GUI

This fixes the test failures introduced by 87a5cf5c, by changing the
default subsystem for arm-pe from 9 (IMAGE_SUBSYSTEM_WINDOWS_CE_GUI) to
2 (IMAGE_SUBSYSTEM_WINDOWS_GUI), which matches what happens with other
PE targets.

As far as I can tell there's no working modern Windows CE toolchain
knocking about anyway, so this change shouldn't inconvenience anyone.