Do not assume kernel is configured with HAVE_SET_TRIGGER

[Ivan-Velickovic]

I believe there are a couple of ARM platforms (the Raspberry Pi 3 is one of them) that have a non-standard interrupt controller. This platforms are configured with HAVE_SET_TRIGGER to false, which means that the kernel invocation seL4_IRQControl_GetTrigger cannot be used.

Right now this isn't a problem for any of the supported platforms by Microkit, but it may be a problem in the future. This depends on seL4/seL4#1073 being solved.