rust: fix host build on x64 Darwin

rust/host fails to compile on macOS running on Intel x64
because the host target triple is autogenerated to be
'arm64-unknown-linux-'. Rust doesn't have such a target triple, thus the
build fails because there are no pre-built artefacts for bootstrapping.

Fix this by setting RUSTC_HOST_ARCH to 'x86_64-apple-darwin' in case
our host is HOST_ARCH=x86_64 and HOST_OS=Darwin.
This fix is based on the existing fix for Apple silicon [1].

Fixes:
  File "/Volumes/pepe/py/openwrt/build_dir/target-x86_64_musl/host/rustc-1.96.0-src/x.py", line 53, in <module>
    bootstrap.main()
    ~~~~~~~~~~~~~~^^
  File "/Volumes/pepe/py/openwrt/build_dir/target-x86_64_musl/host/rustc-1.96.0-src/src/bootstrap/bootstrap.py", line 1418, in main
    bootstrap(args)
    ~~~~~~~~~^^^^^^
  File "/Volumes/pepe/py/openwrt/build_dir/target-x86_64_musl/host/rustc-1.96.0-src/src/bootstrap/bootstrap.py", line 1366, in bootstrap
    build.download_toolchain()
    ~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Volumes/pepe/py/openwrt/build_dir/target-x86_64_musl/host/rustc-1.96.0-src/src/bootstrap/bootstrap.py", line 697, in download_toolchain
    download_component(download_info)
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/Volumes/pepe/py/openwrt/build_dir/target-x86_64_musl/host/rustc-1.96.0-src/src/bootstrap/bootstrap.py", line 529, in download_component
    get(
    ~~~^
        download_info.base_download_url,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<3 lines>...
        verbose=download_info.verbose,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Volumes/pepe/py/openwrt/build_dir/target-x86_64_musl/host/rustc-1.96.0-src/src/bootstrap/bootstrap.py", line 58, in get
    raise RuntimeError(
    ...<6 lines>...
    )
RuntimeError: src/stage0 doesn't contain a checksum for dist/2026-04-16/rust-std-1.95.0-x86_64-unknown-linux-darwin24.6.0.tar.xz. Pre-built artifacts might not be available for this target at this time, see https://doc.rust-lang.org/nightly/rustc/platform-support.html for more information.

[1] https://github.com/openwrt/packages/commit/105fa3920e12f557bdf1fcbc566fc286fb53e319

Signed-off-by: Georgi Valkov <gvalkov@gmail.com>

lang/rust/rust-values.mk

@@ -32,6 +32,8 @@ CARGO_RUSTFLAGS+=-Ctarget-feature=-crt-static $(RUSTC_LDFLAGS)
 ifeq ($(HOST_OS),Darwin)
   ifeq ($(HOST_ARCH),arm64)
     RUSTC_HOST_ARCH:=aarch64-apple-darwin
+  else ifeq ($(HOST_ARCH),x86_64)
+    RUSTC_HOST_ARCH:=x86_64-apple-darwin
   endif
 endif