cmd/compile: update TestStmtLines threshold for riscv64

cmd/compile: update TestStmtLines threshold for riscv64

CL402374 default regabi for riscv64 but TestStmtLines
keep fail trybot within a tiny overlimit (2.006% > 2%).
This CL update this threshold to 3% for riscv64 as an
acceptable temporary solution.

Change-Id: I5c6f37099a76bc048998eb95f49944dbe55492f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/404195
Run-TryBot: mzh <mzh@golangcn.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>