Note: riscv64-nutshell master branch does not support F & D,
its start.S is different from other riscv64 archs

This commit adds a test for external interrupts. The test uses
AXI4IntrGenerator in XiangShan to generate external interrupts.
Since XiangShan does not support external supervisor interrupts, we detect
external interrupts in M mode and pass it to S mode via sip.seip.

dummy: add dummy devices

See merge request projectn/nexus-am!48

apps,fceux: support video without GUI

See merge request projectn/nexus-am!47

Riscv64 noop nanoslite

See merge request projectn/nexus-am!46

Fix audio

See merge request projectn/nexus-am!45

Jyy upstream

See merge request projectn/nexus-am!43

Audio

See merge request projectn/nexus-am!42

* this helps to avoid out-of-memory in fceux, since fceux will repeatly
  call malloc()/free() when copying data to the audio buffer

* this is to support 64-bit divisions in fceux

* ADDCYC() can still maintain soundtimestamp correctly