Add Sstval support

[chenguokai]

Do not merge before backend support!

[poemonsense]

For rocket-chip changes, it's recommended to upstream functional changes to chipsalliance.

[chenguokai]

it's recommended to upstream functional changes to chipsalliance

Sstval feature requires more than just decoder logic to work. Shall we contribute this logic to rocket-chip with backend support? @huxuan0307

[XiangShanRobot]

[Generated by IPC robot]
commit: 853b415

commit	astar	copy_and_run	coremark	gcc	gromacs	lbm	linux	mcf	microbench	milc	namd	povray	wrf	xalancbmk
853b415	1.875	0.461	2.100	1.195	2.904	2.175	2.218	0.918	1.421	1.125	3.611	2.659	2.300	3.176

master branch:

commit	astar	copy_and_run	coremark	gcc	gromacs	lbm	linux	mcf	microbench	milc	namd	povray	wrf	xalancbmk
ef6723f
58a9a40
8f62644	1.862	0.460	2.087	1.204	2.903	2.177	2.331	0.929	1.427	1.138	3.610	2.645	2.297	3.183
f410867	1.874	0.460	2.087	1.208	2.921	2.177	2.361	0.919	1.427	1.138	3.616	2.657	2.296	3.151
ba7cfb6	1.874	0.460	2.087	1.205	2.921	2.177	2.361	0.919	1.427	1.138	3.616	2.657	2.296	3.151
f57f7f2	1.874	0.460	2.087	1.207	2.921	2.177	2.361	0.919	1.427	1.138	3.616	2.657	2.296	3.151
bdccdb7	1.885	0.461	2.100	1.199	2.904	2.175	2.358	0.919	1.421	1.127	3.611	2.647	2.300	3.176
021511b	1.875	0.461	2.100	1.194	2.904	2.175	2.360	0.918	1.421	1.125	3.611	2.660	2.300	3.176
85f365a	1.885	0.461	2.100	1.194	2.904	2.175	2.360	0.919	1.421	1.127	3.611	2.659	2.300	3.176
f541b46	1.875	0.461	2.100	1.200	2.904	2.181	2.218	0.918	1.421	1.125	3.611	2.660	2.300	3.176