Add support for i586-unknown-netbsd as target.

compiler/rustc_llvm/build.rs

@@ -258,6 +258,12 @@ fn main() {
     {
         println!("cargo:rustc-link-lib=z");
     } else if target.contains("netbsd") {
+        // Building for i586 or i686, we need -latomic for 64-bit atomics
+        if target.starts_with("i586")
+           || target.starts_with("i686")
+        {
+            println!("cargo:rustc-link-lib=atomic");
+        }
         println!("cargo:rustc-link-lib=z");
         println!("cargo:rustc-link-lib=execinfo");
     }

compiler/rustc_target/src/spec/i586_unknown_netbsd.rs

@@ -0,0 +1,22 @@
+use crate::spec::{Cc, Lld, LinkerFlavor, StackProbeType, Target, TargetOptions};
+
+pub fn target() -> Target {
+    let mut base = super::netbsd_base::opts();
+    base.cpu = "pentium".into();
+    base.max_atomic_width = Some(64);
+    base.pre_link_args
+        .entry(LinkerFlavor::Gnu(Cc::Yes, Lld::No))
+        .or_default()
+        .push("-m32".into());
+    base.stack_probes = StackProbeType::Call;
+
+    Target {
+        llvm_target: "i586-unknown-netbsdelf".into(),
+        pointer_width: 32,
+        data_layout: "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-\
+            f64:32:64-f80:32-n8:16:32-S128"
+            .into(),
+        arch: "x86".into(),
+        options: TargetOptions { mcount: "__mcount".into(), ..base },
+    }
+}

compiler/rustc_target/src/spec/mod.rs

@@ -1426,6 +1426,7 @@ supported_targets! {
     ("aarch64_be-unknown-netbsd", aarch64_be_unknown_netbsd),
     ("armv6-unknown-netbsd-eabihf", armv6_unknown_netbsd_eabihf),
     ("armv7-unknown-netbsd-eabihf", armv7_unknown_netbsd_eabihf),
+    ("i586-unknown-netbsd", i586_unknown_netbsd),
     ("i686-unknown-netbsd", i686_unknown_netbsd),
     ("powerpc-unknown-netbsd", powerpc_unknown_netbsd),
     ("riscv64gc-unknown-netbsd", riscv64gc_unknown_netbsd),