Auto merge of #2784 - nmeum:riscv64-enotsup, r=Amanieu

Add ENOTSUP constant for riscv64 musl

Previously:

* #2595
* #1971
* #448

but not of these added the constant for riscv64 musl and rustc itself seems to use it nowadays.

src/unix/linux_like/linux/musl/b64/riscv64/mod.rs

@@ -546,6 +546,7 @@ pub const ENOPROTOOPT: ::c_int = 92;
 pub const EPROTONOSUPPORT: ::c_int = 93;
 pub const ESOCKTNOSUPPORT: ::c_int = 94;
 pub const EOPNOTSUPP: ::c_int = 95;
+pub const ENOTSUP: ::c_int = EOPNOTSUPP;
 pub const EPFNOSUPPORT: ::c_int = 96;
 pub const EAFNOSUPPORT: ::c_int = 97;
 pub const EADDRINUSE: ::c_int = 98;