fix(linux): pass sysroot flags to assembly actions

Keep linker-specific -Wl,--sysroot flags limited to link actions while applying
the compiler sysroot to assembly actions.

resolves #141

Author: nradakovic

docs/features.md

@@ -70,7 +70,9 @@ enabled by default.
 - **`output_execpath_flags`** (both) — `-o` for the link output.
 - **`libraries_to_link`** (both) — Handles whole-archive, static, object-file,
   dynamic, and versioned-dynamic library linking.
-- **`sysroot_link_flags`** (Linux) — Adds `--sysroot` / `-Wl,--sysroot` at link.
+- **`sysroot_link_flags`** (Linux) — Adds `--sysroot` / `-Wl,--sysroot` at link,
+  and `--sysroot` for assemble/preprocess-assemble actions (preprocessed `.S`
+  sources need it to resolve sysroot headers).
 
 ### Opt-in link features (Linux)
 These mirror Bazel's legacy features and are guarded, so they are no-ops until

docs/maintenance.md

@@ -53,10 +53,12 @@ feature, though; some behavior is provided by other toolchain wiring:
 - **`gcov`** is provided via `tool_paths` (`gcov_wrapper`).
 - **Sysroot (Linux)** — the sysroot path is passed as `builtin_sysroot` to
   `create_cc_toolchain_config_info`; compile-time header resolution relies on
-  `cxx_builtin_include_directories`, so no `--sysroot` is needed at compile.
-  Link-time `--sysroot` / `-Wl,--sysroot` is emitted by the custom
-  `sysroot_link_flags` feature. There is intentionally no legacy `sysroot`
-  compile feature.
+  `cxx_builtin_include_directories`, so no `--sysroot` is needed for c/cxx
+  compiles. Link-time `--sysroot` / `-Wl,--sysroot` and assembly-time
+  `--sysroot` (assemble / preprocess-assemble; needed for `.S` sources since
+  the extra_*_compile_flags include paths do not apply there) are emitted by
+  the custom `sysroot_link_flags` feature. There is intentionally no legacy
+  `sysroot` compile feature.
 - **Sysroot / system includes (QNX)** — QNX does not use `builtin_sysroot`;
   system include roots come from `cxx_builtin_include_directories` (SDP paths),
   and the SDP environment is injected by the `sdp_env` feature.

features/native/sysroot_link_flags/features.bzl

@@ -1,3 +1,6 @@
+load("@rules_cc//cc/toolchains:feature.bzl", "cc_feature")
+load("@rules_cc//cc/toolchains/args:sysroot.bzl", "cc_sysroot")
+
 # *******************************************************************************
 # Copyright (c) 2026 Contributors to the Eclipse Foundation
 #
@@ -11,9 +14,6 @@
 # SPDX-License-Identifier: Apache-2.0
 # *******************************************************************************
 
-load("@rules_cc//cc/toolchains:feature.bzl", "cc_feature")
-load("@rules_cc//cc/toolchains/args:sysroot.bzl", "cc_sysroot")
-
 def make_sysroot_link_flags(target_os, sysroot):
     """Creates the sysroot_link_flags feature for this toolchain's sysroot directory.
 
@@ -31,8 +31,17 @@ def make_sysroot_link_flags(target_os, sysroot):
             sysroot = sysroot,
         )
 
+        cc_sysroot(
+            name = "sysroot_assembly_flags_args",
+            actions = ["@rules_cc//cc/toolchains/actions:assembly_actions"],
+            sysroot = sysroot,
+        )
+
         cc_feature(
             name = "sysroot_link_flags",
-            args = [":sysroot_link_flags_args"],
+            args = [
+                ":sysroot_link_flags_args",
+                ":sysroot_assembly_flags_args",
+            ],
             feature_name = "sysroot_link_flags",
         )