Use rules_rs platforms for release binaries (#41476)

## What changed

- Map each supported release platform name to its Rust target triple.
- Build multiplatform release binaries against the corresponding `rules_rs`
  platform instead of the LLVM platform definitions.
- Document that `multiplatform_binaries` accepts a subset of the declared
  release platforms.

GitOrigin-RevId: 55e2fb0f92e5fa9e608c10380b1c5d893ca83895

bazel/platforms/release_binaries.bzl

@@ -2,20 +2,23 @@
 
 load("@rules_platform//platform_data:defs.bzl", "platform_data")
 
-PLATFORMS = [
-    "linux_arm64_musl",
-    "linux_amd64_musl",
-    "macos_amd64",
-    "macos_arm64",
-    "windows_amd64",
-    "windows_arm64",
-]
+_PLATFORM_TRIPLES = {
+    "linux_arm64_musl": "aarch64-unknown-linux-musl",
+    "linux_amd64_musl": "x86_64-unknown-linux-musl",
+    "macos_amd64": "x86_64-apple-darwin",
+    "macos_arm64": "aarch64-apple-darwin",
+    "windows_amd64": "x86_64-pc-windows-msvc",
+    "windows_arm64": "aarch64-pc-windows-msvc",
+}
+
+PLATFORMS = _PLATFORM_TRIPLES.keys()
 
 def multiplatform_binaries(name, platforms = PLATFORMS):
+    """Build a binary for a subset of the declared release platforms."""
     for platform in platforms:
         platform_data(
             name = name + "_" + platform,
-            platform = "@llvm//platforms:" + platform,
+            platform = "@rules_rs//rs/platforms:" + _PLATFORM_TRIPLES[platform],
             target = name,
             tags = ["manual"],
         )