deb: resolve cross pkg-config against the target multiarch

In-tree tools locate their libraries with the *host* pkg-config during
cross builds (the kernel's tools/build feature checks derive their
cflags/ldflags from 'pkg-config --cflags/--libs'), whose search path
only covers the build architecture's pkgconfig dirs. Cross builds of
linux-riscv died in rtla's Makefile.config: libtraceevent/libtracefs
were reported missing although the riscv64 -dev packages were
installed, because pkg-config never saw their .pc files.

The host-arch -dev packages that used to make those checks pass came
from the unscoped arch-indep build-dep pass, whose native
re-resolution b34e86d correctly scoped to the host arch — removing the
accidental co-install along with the bug it papered over. Export
PKG_CONFIG_LIBDIR for the target multiarch instead, so the checks
resolve target-arch libraries directly: no native build-dep bloat and
no wrong-arch linking.

src/deb/cross.rs

@@ -62,6 +62,20 @@ pub fn setup_environment(
         .map_err(|e| format!("Invalid UTF-8 in dpkg-architecture output: {e}"))?;
     parse_dpkg_architecture_output(&dpkg_architecture, env);
 
+    // In-tree tools locate their libraries with the *host* pkg-config during
+    // cross builds (the kernel's tools/build feature checks derive their
+    // cflags/ldflags from `pkg-config --cflags/--libs`), whose search path
+    // only covers the build architecture's pkgconfig dirs. Point it at the
+    // target's so `libtraceevent` & co resolve to target-arch libraries:
+    // linux-riscv cross builds die in rtla's Makefile.config otherwise, even
+    // with the target -dev packages installed.
+    if let Some(multiarch) = env.get("DEB_HOST_MULTIARCH").cloned() {
+        env.insert(
+            "PKG_CONFIG_LIBDIR".to_string(),
+            format!("/usr/lib/{multiarch}/pkgconfig:/usr/share/pkgconfig"),
+        );
+    }
+
     env.insert("DEB_BUILD_PROFILES".to_string(), "cross".to_string());
 
     Ok(())
@@ -289,4 +303,25 @@ mod tests {
         );
         assert!(cross_suites("noble", None, "not-a-distro").is_err());
     }
+
+    /// setup_environment exports the target multiarch pkg-config libdir:
+    /// tools' feature checks run the *host* pkg-config, which must find the
+    /// target's .pc files (rtla hard-errors on libtraceevent otherwise,
+    /// failing linux-riscv cross builds despite the target -dev packages
+    /// being installed).
+    #[test]
+    fn test_setup_environment_exports_cross_pkg_config_libdir() {
+        let mut env = HashMap::new();
+        let ctx = Arc::new(Context::new(crate::context::ContextConfig::Local).unwrap());
+        setup_environment(&mut env, "riscv64", ctx).unwrap();
+
+        assert_eq!(
+            env.get("PKG_CONFIG_LIBDIR").map(String::as_str),
+            Some("/usr/lib/riscv64-linux-gnu/pkgconfig:/usr/share/pkgconfig")
+        );
+        assert_eq!(
+            env.get("DEB_BUILD_PROFILES").map(String::as_str),
+            Some("cross")
+        );
+    }
 }