cmd/dist: detect gohostarch on riscv64 hosts

Updates #27532

Change-Id: I66c194499bb7b831b569c66d0736fa7205eedd80
Reviewed-on: https://go-review.googlesource.com/c/go/+/215837
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/cmd/dist/main.go b/src/cmd/dist/main.go
index 3e54915..b8a8c5f 100644
--- a/src/cmd/dist/main.go
+++ b/src/cmd/dist/main.go
@@ -122,6 +122,8 @@
 			if elfIsLittleEndian(os.Args[0]) {
 				gohostarch = "mipsle"
 			}
+		case strings.Contains(out, "riscv64"):
+			gohostarch = "riscv64"
 		case strings.Contains(out, "s390x"):
 			gohostarch = "s390x"
 		case gohostos == "darwin":