Add riscv64 into BAZEL_GOARCH_CONSTRAINTS (#4507)

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**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

Fix some Bazel matching issues on riscv64

**Which issues(s) does this PR fix?**

Fixes #4053

**Other notes for review**

The riscv64 architecture has been widely developed and adopted recently,
and optimizing support for riscv64 is very meaningful as it can provide
better riscv64 support for downstream users.

For example, when I was using `bazeldnf`, due to some limitations in the
current repository, I couldn't use `bazeldnf` directly. The specific
reason is this:
https://github.com/rmohr/bazeldnf/blob/main/bazeldnf/platforms.bzl. This
depends on the current repository's `BAZEL_GOARCH_CONSTRAINTS`, which
currently lacks riscv64 support, so the riscv64 version of bazeldnf
cannot be used directly. If this PR is merged and released, downstream
users will be able to get good riscv64 support.

A PR quite similar to this one is
#3336. I think the
situation with riscv64 is quite similar to that case. The Go upstream
also provides good support for riscv64.


**Other Info**
Co-authored by: [email protected];

Author: ffgan

go/private/mode.bzl

@@ -105,6 +105,7 @@ _LINK_PLUGIN_PLATFORMS = {
     "linux/386": None,
     "linux/s390x": None,
     "linux/ppc64le": None,
+    "linux/riscv64": None,
     "android/amd64": None,
     "android/arm": None,
     "android/arm64": None,
@@ -122,6 +123,7 @@ _LINK_PIE_PLATFORMS = {
     "linux/386": None,
     "linux/s390x": None,
     "linux/ppc64le": None,
+    "linux/riscv64": None,
     "android/amd64": None,
     "android/arm": None,
     "android/arm64": None,

go/private/platforms.bzl

@@ -34,6 +34,7 @@ BAZEL_GOARCH_CONSTRAINTS = {
     "ppc64": "@platforms//cpu:ppc",
     "ppc64le": "@platforms//cpu:ppc64le",
     "s390x": "@platforms//cpu:s390x",
+    "riscv64": "@platforms//cpu:riscv64",
 }
 
 GOOS_GOARCH = (