openssl: specify the ABI explicitly on mips64

When *not* cross-compiling, OpenSSL will not attempt to detect the
host ABI.  For mips64, the OpenSSL authors have chosen to assume that
the n32 ABI is used.

Since nixpkgs knows the correct ABI based on stdenv.hostPlatform,
let's pass this information to OpenSSL explicitly.

At the moment (bootstrappable) nixpkgs on mips64 can only be used with
the n64 ABI due to the fact that boost-context (required by nix) does
not support the n32 ABI.  Without this commit the openssl expression
can be cross-compiled to a mips64 host, but a mips64 host cannot
self-compile the expression due to OpenSSL's incorrect assumption.

#165746 (review)

pkgs/development/libraries/openssl/default.nix

@@ -69,6 +69,12 @@ let
         x86_64-linux = "./Configure linux-x86_64";
         x86_64-solaris = "./Configure solaris64-x86_64-gcc";
         riscv64-linux = "./Configure linux64-riscv64";
+        mips64el-linux =
+          if stdenv.hostPlatform.isMips64n64
+          then "./Configure linux64-mips64"
+          else if stdenv.hostPlatform.isMips64n32
+          then "./Configure linux-mips64"
+          else throw "unsupported ABI for ${stdenv.hostPlatform.system}";
       }.${stdenv.hostPlatform.system} or (
         if stdenv.hostPlatform == stdenv.buildPlatform
           then "./config"