[infra] Upload RISCV64 SDK.

gclient sync will need a prebuilt SDK on RISCV64 hosts.

Bug: https://github.com/dart-lang/sdk/issues/48545
Change-Id: I90358d27c66e1ec8228419ae0fa463c98e2cb5e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237627
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

tools/bots/test_matrix.json

@@ -2986,7 +2986,7 @@
       },
       "steps": [
         {
-          "name": "build dart",
+          "name": "build dart (clang builds)",
           "script": "tools/build.py",
           "arguments": [
             "--arch=ia32,x64,arm,arm64",
@@ -2995,11 +2995,22 @@
             "create_sdk"
           ]
         },
+        {
+          "name": "build dart (gcc builds)",
+          "script": "tools/build.py",
+          "arguments": [
+            "--no-clang",
+            "--arch=riscv64",
+            "--mode=release",
+            "--check-clean",
+            "create_sdk"
+          ]
+        },
         {
           "name": "upload sdk",
           "script": "tools/bots/dart_sdk.py",
           "arguments": [
-            "--arch=ia32,x64,arm,arm64"
+            "--arch=ia32,x64,arm,arm64,riscv64"
           ]
         },
         {