Add powerpc64-unknown-openbsd support

Index: compiler/rustc_target/src/spec/powerpc64_unknown_openbsd.rs
--- compiler/rustc_target/src/spec/powerpc64_unknown_openbsd.rs.orig
+++ compiler/rustc_target/src/spec/powerpc64_unknown_openbsd.rs
@@ -0,0 +1,17 @@
+use crate::abi::Endian;
+use crate::spec::{LinkerFlavor, Target, TargetOptions};
+
+pub fn target() -> Target {
+    let mut base = super::openbsd_base::opts();
+    base.cpu = "ppc64".into();
+    base.pre_link_args.entry(LinkerFlavor::Gcc).or_default().push("-m64".into());
+    base.max_atomic_width = Some(64);
+
+    Target {
+        llvm_target: "powerpc64-unknown-openbsd".into(),
+        pointer_width: 64,
+        data_layout: "E-m:e-i64:64-n32:64".into(),
+        arch: "powerpc64".into(),
+        options: TargetOptions { endian: Endian::Big, mcount: "_mcount".into(), ..base },
+    }
+}
