fix(toolchain): 固定的 host 工具链装不上时,回退到已经在本机可用的 target 工具链

`ci-aarch64-fresh-install` 在 aarch64 上构建 xlings 时死在:

    [error] download failed for xim:gcc@16.1.0: HTTP 404
    error: host toolchain for build.mcpp ('gcc@16.1.0'): ...

链条是:图里有包带 build.mcpp ⇒ 需要 HOST 编译器 ⇒ 取 `[toolchain] default`
⇒ 那是 `gcc@16.1.0`,而 xim 的 gcc 声明 `archs = { "x86_64" }` ⇒ 404。

`[toolchain]` 的键是 OS,**没有 arch 这根轴**(#367),所以 aarch64 问的和
x86_64 问的是同一个包。结果是:凡是依赖图里出现 build.mcpp 的工程,在 aarch64
上就是死路。

但这台机器上**已经有**一个能用的编译器:当解析出的 target 与 host 只差 libc
环境(aarch64-linux-musl 跑在 aarch64 Linux 上),target 工具链就是本机原生的
—— 它在这里能跑,它产出的二进制在这里也能跑,拿它编译一个构建程序完全成立。

因此:固定的 host 工具链取不到时,若 arch 与 os 与 host 相同,就用 target 工具
链,并发一条命名双方的告警。成功路径一字未改;此前的死路变成一次可解释的降级。

#367 记录的仍是根因(host 工具链选择缺 arch 轴),这条只是让它不再是绝路。

Author: speak-agent

src/build/prepare.cppm

@@ -1812,6 +1812,34 @@ prepare_build(bool print_fingerprint,
         mcpp::fetcher::InstallProgressHandler progress;
         auto payload = fetcher.resolve_xpkg_path(pkg.target(), /*autoInstall=*/true, &progress);
         if (!payload) {
+            // The pinned host toolchain is not obtainable here. Before giving
+            // up, ask whether we already HAVE a compiler that runs on this
+            // machine: when the resolved target differs from the host only in
+            // its libc environment (aarch64-linux-musl on an aarch64 Linux
+            // host), the target toolchain is native — it runs here and the
+            // binaries it produces run here — so it is a perfectly good
+            // compiler for a build program.
+            //
+            // Without this, a whole architecture is a dead end for any project
+            // whose graph contains a build.mcpp: `[toolchain]` is keyed by OS
+            // with no arch axis (#367), so `default = "gcc@16.1.0"` is what
+            // aarch64 asks for too, and that package declares
+            // `archs = { "x86_64" }` — the download 404s and the build stops.
+            // A toolchain that is already on disk and already correct should
+            // not be unusable because a second, redundant one is missing.
+            auto hostT = mcpp::toolchain::triple::host_triple();
+            auto tgtT  = mcpp::toolchain::triple::parse(overrides.target_triple);
+            if (tgtT && tgtT->arch == hostT.arch && tgtT->os == hostT.os) {
+                mcpp::diag::warning("toolchain/host-fallback", std::format(
+                    "the pinned host toolchain '{}' could not be provisioned "
+                    "({}), so build.mcpp will be compiled with the target "
+                    "toolchain instead — it is native here ({} vs host {}). "
+                    "Pin a host toolchain that exists on this architecture to "
+                    "silence this.",
+                    *tcSpec, payload.error().message, tgtT->str(), hostT.str()));
+                hostTcCache = std::pair{explicit_compiler, *tc};
+                return *hostTcCache;
+            }
             return std::unexpected(std::format(
                 "host toolchain for build.mcpp ('{}'): {}", *tcSpec,
                 payload.error().message));