runtime: use regabi for riscv64 cputicks

runtime: use regabi for riscv64 cputicks

goos: linux
goarch: riscv64
pkg: runtime
cpu: Spacemit(R) X60
                     │ select.old.log │           select.new.log           │
                     │     sec/op     │   sec/op     vs base               │
SelectUncontended         490.5n ± 0%   486.8n ± 0%  -0.77% (p=0.000 n=10)
SelectSyncContended       2.754µ ± 0%   2.726µ ± 0%  -1.02% (p=0.000 n=10)
SelectAsyncContended      488.2n ± 0%   484.2n ± 0%  -0.84% (p=0.000 n=10)
SelectNonblock            112.2n ± 0%   111.5n ± 0%  -0.58% (p=0.000 n=10)
SelectProdCons            1.420µ ± 0%   1.417µ ± 0%       ~ (p=0.069 n=10)
GoroutineSelect           10.79m ± 3%   10.74m ± 3%       ~ (p=0.529 n=10)
geomean                   3.228µ        3.208µ       -0.63%

Change-Id: Idb519ef8b2872284dca6dbf1cf94c3fff65bfd37
Reviewed-on: https://go-review.googlesource.com/c/go/+/610095
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Joel Sing <joel@sing.id.au>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>