Auto merge of #2537 - kanavin:add-riscv, r=Amanieu

Add base definitions for riscv64 + musl

This is picking up the stale MR that was closed for inactivity:
#1994

Author: bors

ci/build.sh

@@ -141,6 +141,7 @@ armv5te-unknown-linux-gnueabi \
 armv5te-unknown-linux-musleabi \
 i686-pc-windows-gnu \
 riscv64gc-unknown-linux-gnu \
+riscv64gc-unknown-linux-musl \
 wasm32-wasi \
 x86_64-fortanix-unknown-sgx \
 x86_64-fuchsia \

src/unix/linux_like/linux/musl/b64/mod.rs

@@ -163,6 +163,9 @@ cfg_if! {
     } else if #[cfg(any(target_arch = "x86_64"))] {
         mod x86_64;
         pub use self::x86_64::*;
+    } else if #[cfg(any(target_arch = "riscv64"))] {
+        mod riscv64;
+        pub use self::riscv64::*;
     } else {
         // Unknown target_arch
     }