add sparc64-unknown-openbsd target

on OpenBSD, some architectures relies on libc++ (from LLVM) and some
others on libestdc++ (particular version of libstdc++ from GCC).

sparc64-unknown-openbsd needs libestdc++ and libgcc (as x86_64 some
years ago). Reintroduce the support of them for openbsd, only for
sparc64 arch. Some others architectures on OpenBSD could use them too.

src/librustc_llvm/build.rs

@@ -250,8 +250,11 @@ fn main() {
     let llvm_use_libcxx = env::var_os("LLVM_USE_LIBCXX");
 
     let stdcppname = if target.contains("openbsd") {
-        // llvm-config on OpenBSD doesn't mention stdlib=libc++
-        "c++"
+        if target.contains("sparc64") {
+            "estdc++"
+        } else {
+            "c++"
+        }
     } else if target.contains("freebsd") {
         "c++"
     } else if target.contains("darwin") {

src/librustc_target/spec/mod.rs

@@ -395,6 +395,7 @@ supported_targets! {
 
     ("aarch64-unknown-openbsd", aarch64_unknown_openbsd),
     ("i686-unknown-openbsd", i686_unknown_openbsd),
+    ("sparc64-unknown-openbsd", sparc64_unknown_openbsd),
     ("x86_64-unknown-openbsd", x86_64_unknown_openbsd),
 
     ("aarch64-unknown-netbsd", aarch64_unknown_netbsd),

src/librustc_target/spec/sparc64_unknown_openbsd.rs

@@ -0,0 +1,22 @@
+use crate::spec::{LinkerFlavor, Target, TargetResult};
+
+pub fn target() -> TargetResult {
+    let mut base = super::openbsd_base::opts();
+    base.cpu = "v9".to_string();
+    base.pre_link_args.get_mut(&LinkerFlavor::Gcc).unwrap().push("-m64".to_string());
+    base.max_atomic_width = Some(64);
+
+    Ok(Target {
+        llvm_target: "sparc64-unknown-openbsd".to_string(),
+        target_endian: "big".to_string(),
+        target_pointer_width: "64".to_string(),
+        target_c_int_width: "32".to_string(),
+        data_layout: "E-m:e-i64:64-n32:64-S128".to_string(),
+        arch: "sparc64".to_string(),
+        target_os: "openbsd".to_string(),
+        target_env: String::new(),
+        target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
+        options: base,
+    })
+}

src/libunwind/build.rs

@@ -23,7 +23,11 @@ fn main() {
     } else if target.contains("netbsd") {
         println!("cargo:rustc-link-lib=gcc_s");
     } else if target.contains("openbsd") {
-        println!("cargo:rustc-link-lib=c++abi");
+        if target.contains("sparc64") {
+            println!("cargo:rustc-link-lib=gcc");
+        } else {
+            println!("cargo:rustc-link-lib=c++abi");
+        }
     } else if target.contains("solaris") {
         println!("cargo:rustc-link-lib=gcc_s");
     } else if target.contains("dragonfly") {