Rollup merge of #101025 - semarie:openbsd-archs, r=petrochenkov

Add tier-3 support for powerpc64 and riscv64 openbsd

# powerpc64
- MCP for [powerpc64-unknown-openbsd tier-3 support](https://github.com/rust-lang/compiler-team/issues/551)
- only need to add spec definition in rustc_target

# riscv64
- MCP for [riscv64-unknown-openbsd tier-3 support](https://github.com/rust-lang/compiler-team/issues/552)
- add spec definition in rustc_target
- follow freebsd about avoiding linking with `libatomic`

compiler/rustc_llvm/build.rs

@@ -342,10 +342,10 @@ fn main() {
     };
 
     // RISC-V GCC erroneously requires libatomic for sub-word
-    // atomic operations. FreeBSD uses Clang as its system
+    // atomic operations. Some BSD uses Clang as its system
     // compiler and provides no libatomic in its base system so
     // does not want this.
-    if !target.contains("freebsd") && target.starts_with("riscv") {
+    if target.starts_with("riscv") && !target.contains("freebsd") && !target.contains("openbsd") {
         println!("cargo:rustc-link-lib=atomic");
     }