In this commit, we add License for XiangShan project. 

This commit adds support for using Synopsys VCS to simulate SimTop.
Difftest is also supported.

For now, we use src/test/vsrc/vcs/top.v as the top-level module.
In the future, we may support VCS slave mode for better scalability.

* difftest: use DPI-C to refactor difftest

In this commit, difftest is refactored with DPI-C calls.
There're a few reasons:
(1) From Verilator's manual, DPI-C calls should be more efficient than accessing from dut_ptr.
(2) DPI-C is cross-platform (Verilator, VCS, ...)
(3) difftest APIs are splited from emu.cpp to possibly support more backend platforms
(NEMU, Spike, ...)

The performance at this commit is quite slower than the original emu.
Performance issues will be fixed later.

* [WIP] SimTop: try to use 'XSTop' as soc

* CircularQueuePtr: ues F-bounded polymorphis instead implict helper

* Refactor parameters & Clean up code

* difftest: support basic difftest

* Support diffetst in new sim top

* Difftest; convert recode fmt to ieee754 when comparing fp regs

* Difftest: pass sign-ext pc to dpic functions && fix exception pc

* Debug: add int/exc inst wb to debug queue

* Difftest: pass sign-ext pc to dpic functions && fix exception pc

* Difftest: fix naive commit num limit
Co-authored-by: NYinan Xu <xuyinan1997@gmail.com>
Co-authored-by: NWilliam Wang <zeweiwang@outlook.com>