Add RISC-V 64bit support

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>
--- a/third_party/boringssl/include/openssl/base.h
+++ b/third_party/boringssl/include/openssl/base.h
@@ -108,6 +108,9 @@ extern "C" {
 #elif defined(__mips__) && defined(__LP64__)
 #define OPENSSL_64_BIT
 #define OPENSSL_MIPS64
+#elif defined(__riscv) && defined(__riscv_xlen)
+#define OPENSSL_64_BIT
+#define OPENSSL_RISCV64
 #elif defined(__pnacl__)
 #define OPENSSL_32_BIT
 #define OPENSSL_PNACL