Add support for i586-unknown-netbsd as target.

This restricts instructions to those offered by Pentium,
to support e.g. AMD Geode.

There is already an entry for this target in the NetBSD
platform support page at

  src/doc/rustc/src/platform-support/netbsd.md

...so this should forestall its removal.

Additional fixes are needed for some vendored modules, this
is the changes in the rust compiler core itself.

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 @@ fn $module() {
     ("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),