replace default YIELDING by sched_yield with "nop" for RISCV64

[CheryDan]

#4985 when testing the nop YIELDING in bench_symm at SG2042 by perf ,it got the performance improvement of cache and branch misses .

perf arg	sched_yield	nop	ratio
duration_time	858,189,070	846,633,123	0.986534498
task-clock	50,732.36	50,121.99	0.987968823
cycles	101,461,057,914	100,240,372,989	0.987968932
instructions	68,463,403,439	62,824,853,943	0.917641408
cache-references	1,180,192,443	515,655,574	0.436924992
cache-misses	1,180,195,818	515,658,928	0.436926585
branches	6,795,994,978	6,675,145,698	0.982217574
branch-misses	67,157,179	4,502,002	0.067036794
L1-dcache-loads	19,105,443,229	13,524,142,477	0.707868554
L1-dcache-load-misses	55,661,342	9,939,946	0.178578986
LLC-load-misses	13,924,594	13,411,098	0.96312309
LLC-loads	1,180,208,048	515,670,980	0.436932269

[martin-frbg]

thank you