fpga_diff: add UVHS XiangShan and GBus support

[xiaokamikami]

This PR adds optional GBus transport support to the existing FPGA-Diff flow while preserving XDMA as the default interface.

• Add DIFFTEST_HOSTIF selection for XDMA or GBUS.
• Add the UVHS GBus host adapter and conditional endpoint integration.
• Bundle the verified GBus IP DCPs, stubs, runtime library, headers, and installation checks.
• Register the GBus adapter in the FPGA RTL file list.
• Keep existing XDMA behavior unchanged when DIFFTEST_HOSTIF=XDMA.
  The UVHS XiangShan GBus configuration has been validated on hardware with FPGA-Diff reaching HIT GOOD TRAP.