test: re-enable atomic intrinsic related tests on riscv64

riscv64 now has atomic intrinsics, so re-enable the atomic intrinsic tests.

Fixes #36765

Change-Id: I838f27570a94d7fa5774c43f1ca5f4df2ca104cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/223560


Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>