add support for riscv64 arch

From: https://github.com/etcd-io/bbolt/pull/159/files
--- /dev/null
+++ b/bolt_riscv64.go
@@ -0,0 +1,10 @@
+//go:build riscv64
+// +build riscv64
+
+package bbolt
+
+// maxMapSize represents the largest mmap size supported by Bolt.
+const maxMapSize = 0xFFFFFFFFFFFF // 256TB
+
+// maxAllocSize is the size used when creating array pointers.
+const maxAllocSize = 0x7FFFFFFF