Rollup merge of rust-lang#48489 - glaubitz:x32-linux, r=alexcrichton

bootstrap: Add openssl configuration for x86_64-unknown-linux-gnux32

OpenSSL provides a native configuration for x86_64-unknown-linux-gnux32:

> https://github.com/openssl/openssl/blob/master/Configurations/10-main.conf#L810

Let's use it.

Author: kennytm

src/bootstrap/native.rs

@@ -492,6 +492,7 @@ impl Step for Openssl {
             "x86_64-unknown-freebsd" => "BSD-x86_64",
             "x86_64-unknown-dragonfly" => "BSD-x86_64",
             "x86_64-unknown-linux-gnu" => "linux-x86_64",
+            "x86_64-unknown-linux-gnux32" => "linux-x32",
             "x86_64-unknown-linux-musl" => "linux-x86_64",
             "x86_64-unknown-netbsd" => "BSD-x86_64",
             _ => panic!("don't know how to configure OpenSSL for {}", target),