Detect OpenHarmony Rust target

The OpenHarmony SDK contains a CMake toolchain file,
which among others sets CMAKE_OHOS_ARCH_ABI.
We can use this to automatically infer the correct Rust target.

The SDK can be obtained from the release notes on gitee:
https://gitee.com/openharmony/docs/tree/master/en/release-notes

cmake/FindRust.cmake

@@ -713,6 +713,19 @@ if (NOT Rust_CARGO_TARGET_CACHED)
         if (_Rust_ANDROID_TARGET)
             set(Rust_CARGO_TARGET_CACHED "${_Rust_ANDROID_TARGET}" CACHE STRING "Target triple")
         endif()
+    elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "OHOS")
+        if(CMAKE_OHOS_ARCH_ABI STREQUAL arm64-v8a)
+            set(_RUST_OHOS_TARGET aarch64-unknown-linux-ohos)
+        elseif(CMAKE_OHOS_ARCH_ABI STREQUAL armeabi-v7a)
+            set(_RUST_OHOS_TARGET armv7-unknown-linux-ohos)
+        elseif(CMAKE_OHOS_ARCH_ABI STREQUAL x86_64)
+            set(_RUST_OHOS_TARGET x86_64-unknown-linux-ohos)
+        else()
+            message(WARNING "unrecognized OHOS architecture: ${OHOS_ARCH}")
+        endif()
+        if(_RUST_OHOS_TARGET)
+            set(Rust_CARGO_TARGET_CACHED "${_RUST_OHOS_TARGET}" CACHE STRING "Target triple")
+        endif()
     endif()
     # Fallback to the default host target
     if(NOT Rust_CARGO_TARGET_CACHED)