Rollup merge of rust-lang#117170 - he32:netbsd-i586, r=bjorn3

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 +258,14 @@ fn main() {
     {
         println!("cargo:rustc-link-lib=z");
     } else if target.contains("netbsd") {
+        // On NetBSD/i386, gcc and g++ is built for i486 (to maximize backward compat)
+        // However, LLVM insists on using 64-bit atomics.
+        // This gives rise to a need to link rust itself with -latomic for these targets
+        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),