riscv syscall performance regression

[Wu, Fei]

Hi All,

I am doing some performance regression testing on a sophgo machine, the
unixbench syscall benchmark drops 14% from 6.1 to 6.6. This change
should be due to commit f0bddf50 riscv: entry: Convert to generic entry.
I know it's a tradeoff, just checking if it's been discussed already and
any improvement can be done.

The unixbench benchmark I used is:
	$ ./syscall 10 getpid

The dynamic instruction count per syscall is increased from ~200 to
~250, this should be the key factor so I switch to test it on system
QEMU to avoid porting different versions on sophgo, and use plugin
libinsn.so to count the instructions. There are a few background noises
during test but the impact should be limited. This is dyninst count per
syscall I got:

* commit d0db02c6 (right before the change): ~200
* commit f0bddf50 (the change): ~250
* commit ffd2cb6b (latest upstream): ~250

Any comment?

Thanks,
Fei.