Disable MADV_SOFT_OFFLINE on riscv for now.

Upstream fix submitted in rust-lang/libc#2391.

Author: sunfishcode

src/imp/libc/io/poll_fd.rs

@@ -33,7 +33,7 @@ bitflags! {
         const NVAL = libc::POLLNVAL;
         /// `POLLRDHUP`
         // TODO: Submitted to upstream libc:
-        // <https://github.com/rust-lang/libc/pull/2247>
+        // <https://github.com/rust-lang/libc/pull/2390>
         #[cfg(not(any(target_arch = "sparc", target_arch = "sparc64")))]
         const RDHUP = 0x2000;
     }

src/imp/libc/io/types.rs

@@ -365,7 +365,11 @@ pub enum Advice {
     #[cfg(any(target_os = "android", target_os = "linux"))]
     LinuxHwPoison = libc::MADV_HWPOISON,
     /// `MADV_SOFT_OFFLINE`
-    #[cfg(any(target_os = "android", target_os = "linux"))]
+    // TODO: Enable riscv once <https://github.com/rust-lang/libc/pull/2391> lands.
+    #[cfg(all(
+        not(target_arch = "riscv64"),
+        any(target_os = "android", target_os = "linux")
+    ))]
     LinuxSoftOffline = libc::MADV_SOFT_OFFLINE,
     /// `MADV_MERGEABLE`
     #[cfg(any(target_os = "android", target_os = "linux"))]