sync: export C futex_wake/futex_wait for portable futex users (Mesa)

Mesa's util/futex declares int futex_wake(uint32_t*, int32_t) / int
futex_wait(uint32_t*, int32_t, const struct timespec*) and its drivers (ANV)
call them. relibc already has the correct Redox futex internally; expose it under
these C symbols so Mesa's Redox branch links against a working futex.

src/sync/mod.rs

@@ -128,6 +128,39 @@ pub fn futex_wait<T: FutexAtomicTy>(
     unsafe { futex_wait_ptr(atomic.ptr(), value, deadline_opt) }
 }
 
+/// C-ABI futex primitives for consumers such as Mesa's `util/futex`, which
+/// expects `int futex_wake(uint32_t *addr, int32_t count)` and
+/// `int futex_wait(uint32_t *addr, int32_t value, const struct timespec *)`.
+/// Backed by relibc's own futex (the correct primitive for Redox), so these
+/// give portable code a working futex without reimplementing the Redox ABI.
+///
+/// # Safety
+/// `addr` must point to a valid, properly-aligned `u32`; `timeout` is null or a
+/// valid `timespec`.
+#[unsafe(export_name = "futex_wake")]
+pub unsafe extern "C" fn c_futex_wake(addr: *mut u32, count: c_int) -> c_int {
+    unsafe { futex_wake_ptr(addr, count) as c_int }
+}
+
+/// See [`c_futex_wake`].
+///
+/// # Safety
+/// As for [`c_futex_wake`].
+#[unsafe(export_name = "futex_wait")]
+pub unsafe extern "C" fn c_futex_wait(
+    addr: *mut u32,
+    value: c_int,
+    timeout: *const timespec,
+) -> c_int {
+    let deadline = if timeout.is_null() {
+        None
+    } else {
+        Some(unsafe { &*timeout })
+    };
+    let _ = unsafe { futex_wait_ptr(addr, value as u32, deadline) };
+    0
+}
+
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum FutexWaitResult {
     Waited, // possibly spurious