linux-build: port/shim/sched.h routes C++ TUs to the host <sched.h>

The decomp's src/sched.h (N64 OSSc* scheduler) is on the include path and
shadows the system header. libstdc++'s <thread>/<mutex> gthr headers in the
fast3d C++ TUs #include <sched.h> and need the real one (cpu_set_t,
sched_yield) -- with -std=gnu++20 libstdc++ defines _GNU_SOURCE so
<pthread.h> also references cpu_set_t. New shim: C++ -> generated
hostsched.h (absolute path, hoststring pattern); C -> #include_next so
src/sched.h still wins as before. Windows build verified green (240/241).

CMakeLists.txt

@@ -152,6 +152,15 @@ configure_file(
   "${CMAKE_SOURCE_DIR}/port/include/hoststring.h.in"
   "${CMAKE_BINARY_DIR}/port/include/hoststring.h" @ONLY)
 
+# Host <sched.h> path for port/shim/sched.h. The decomp's src/sched.h is on
+# the include path and shadows the system header; libstdc++'s <thread>/gthr
+# headers in the fast3d C++ TUs need the real <sched.h> (cpu_set_t,
+# sched_yield). Both MinGW and Linux keep <sched.h> at bin/../include.
+get_filename_component(PORT_HOST_SCHED_H "${_PORT_GCC_DIR}/../include/sched.h" ABSOLUTE)
+configure_file(
+  "${CMAKE_SOURCE_DIR}/port/include/hostsched.h.in"
+  "${CMAKE_BINARY_DIR}/port/include/hostsched.h" @ONLY)
+
 # Host <stdlib.h> path for port/shim/stdlib.h. The N64 include/stdlib.h
 # typedefs lldiv_t/ldiv_t, which conflict with the host's when both land in
 # one C++ TU (fast3d): MinGW's own sec_api/stdlib_s.h does `#include

port/include/hostsched.h.in

@@ -0,0 +1,9 @@
+#ifndef PORT_HOSTSCHED_H
+#define PORT_HOSTSCHED_H
+/* Generated by CMake from hostsched.h.in — do not edit.
+ * Includes the HOST <sched.h> by absolute path so that port/shim/sched.h can
+ * give C++ TUs (fast3d, libstdc++'s <thread>/gthr headers) the real
+ * <sched.h> — cpu_set_t, sched_yield — instead of the decomp's src/sched.h,
+ * which sits on the include path and would otherwise shadow it. */
+#include "@PORT_HOST_SCHED_H@"
+#endif

port/shim/sched.h

@@ -0,0 +1,18 @@
+/*
+ * PC port shim for <sched.h>.
+ *
+ * The decomp ships src/sched.h (the N64 OSSc* scheduler client interface),
+ * which is on the compiler include path. libstdc++'s <thread> / gthr headers
+ * (pulled into the fast3d C++ TUs) do `#include <sched.h>` and need the HOST
+ * header — cpu_set_t, sched_yield — so C++ TUs must be routed to the real
+ * <sched.h> by absolute path (generated hostsched.h, same trick as
+ * port/shim/string.h). C TUs keep resolving <sched.h> to the decomp's own
+ * src/sched.h, exactly as on the console.
+ *
+ * Inert in the N64 build (port/shim is not on its include path).
+ */
+#if defined(__cplusplus)
+#include "hostsched.h"
+#else
+#include_next <sched.h>
+#endif