feat: opt a target triple's stdlib out of the bundled libc

rustc puts the prebuilt stdlib's `self-contained` directory on the linker
search path ahead of the CC toolchain's `-L`, so `-lc` always resolves to
the libc rustc ships. A workspace that builds its own sysroot -- a distro,
an OS image, an embedded target -- silently links the wrong libc, and any
patch it carries in that libc never reaches a Rust binary.

`-C link-self-contained=no` does not help: it leaves the search path in
place (verified on 1.95.0, byte-identical output). The files have to leave
the sandbox instead, so the stdlib repo grows a second filegroup without
the archives, and `declare_rustc_toolchains` grows a triple list that
selects it.

The CRT objects stay in the filtered filegroup. Remove the directory
altogether and rustc stops using self-contained CRT, emitting bare
`rcrt1.o` / `crti.o` names that the clang driver cannot resolve.

Author: lsjostro

rs/private/stdlib_repository.bzl

@@ -20,6 +20,26 @@ rust_stdlib_filegroup(
     visibility = ["//visibility:public"],
 )
 
+# The same stdlib without the libc and CRT archives rustc bundles beside it.
+# rustc searches `self-contained` ahead of the CC toolchain's -L, so dropping
+# the archives there lets `-lc` fall through to the platform sysroot.  The CRT
+# objects stay: rustc names bare crt files the driver cannot find when the
+# directory is absent altogether.
+rust_stdlib_filegroup(
+    name = "rust_std-{target_triple}-external-libc",
+    srcs = glob(
+        [
+            "lib/rustlib/{target_triple}/lib/*.rlib",
+            "lib/rustlib/{target_triple}/lib/*.rmeta",
+            "lib/rustlib/{target_triple}/lib/*{dylib_ext}*",
+            "lib/rustlib/{target_triple}/lib/*{staticlib_ext}",
+            "lib/rustlib/{target_triple}/lib/self-contained/*.o",
+        ],
+        allow_empty = True,
+    ),
+    visibility = ["//visibility:public"],
+)
+
 alias(
     name = "rust_lib-{target_triple}",
     actual = "rust_std-{target_triple}",

rs/toolchains/declare_rustc_toolchains.bzl

@@ -42,7 +42,8 @@ def declare_rustc_toolchains(
         rust_objcopy = None,
         rust_lld = None,
         bpf_linker = None,
-        rust_std = None):
+        rust_std = None,
+        external_libc_triples = []):
     """Declares generated or custom Rust compiler toolchains.
 
     Args:
@@ -64,6 +65,9 @@ def declare_rustc_toolchains(
       rust_lld: Optional rust-lld label or labels keyed by execution triple.
       bpf_linker: Optional bpf-linker label or labels keyed by execution triple.
       rust_std: Optional standard-library label or labels keyed by target triple.
+      external_libc_triples: Target triples whose standard library should link
+        the platform's libc instead of the copy rustc bundles. Ignored for a
+        triple whose standard library is given explicitly in `rust_std`.
     """
     if type(rustc) == "dict":
         for exec_triple in rustc:
@@ -99,6 +103,8 @@ def declare_rustc_toolchains(
         stdlib_repo = "rust_stdlib_%s_%s" % (target_key, version_key)
         if target_triple in SUPPORTED_TIER_3_TRIPLES:
             default_rust_std = "@rustc_src_" + version_key + "//src:rust_std"
+        elif target_triple in external_libc_triples:
+            default_rust_std = "@%s//:rust_std-%s-external-libc" % (stdlib_repo, target_triple)
         else:
             default_rust_std = "@%s//:rust_std-%s" % (stdlib_repo, target_triple)
         rust_std_select[config_label] = _component(rust_std, target_triple, default_rust_std)