add riscv64gc-unknown-openbsd support (target riscv64-unknown-openbsd on OpenBSD)

- add platform-support documentation
- add riscv64gc-unknown-openbsd spec
- do not try to link with -latomic on openbsd

compiler/rustc_target/src/spec/mod.rs

@@ -895,6 +895,7 @@ supported_targets! {
     ("i686-unknown-openbsd", i686_unknown_openbsd),
     ("powerpc-unknown-openbsd", powerpc_unknown_openbsd),
     ("powerpc64-unknown-openbsd", powerpc64_unknown_openbsd),
+    ("riscv64gc-unknown-openbsd", riscv64gc_unknown_openbsd),
     ("sparc64-unknown-openbsd", sparc64_unknown_openbsd),
     ("x86_64-unknown-openbsd", x86_64_unknown_openbsd),
 

compiler/rustc_target/src/spec/riscv64gc_unknown_openbsd.rs

@@ -0,0 +1,18 @@
+use crate::spec::{CodeModel, Target, TargetOptions};
+
+pub fn target() -> Target {
+    Target {
+        llvm_target: "riscv64-unknown-openbsd".into(),
+        pointer_width: 64,
+        data_layout: "e-m:e-p:64:64-i64:64-i128:128-n64-S128".into(),
+        arch: "riscv64".into(),
+        options: TargetOptions {
+            code_model: Some(CodeModel::Medium),
+            cpu: "generic-rv64".into(),
+            features: "+m,+a,+f,+d,+c".into(),
+            llvm_abiname: "lp64d".into(),
+            max_atomic_width: Some(64),
+            ..super::openbsd_base::opts()
+        },
+    }
+}