Remove unsafe ScopedSigaction

ScopedSigaction is not async-signal-safe, so it cannot be used after fork.
Replace it with plain sigaction, which is safe. This is in a unique child
anyways, so it doesn't need any cleanup.

PiperOrigin-RevId: 237102411
Change-Id: I5c6ea373bbac67b9c4db204ceb1db62d338d9178

test/syscalls/linux/kill.cc

@@ -70,7 +70,8 @@ TEST(KillTest, CanKillAllPIDs) {
     sa.sa_sigaction = SigHandler;
     sigfillset(&sa.sa_mask);
     sa.sa_flags = SA_SIGINFO;
-    auto cleanup = ScopedSigaction(SIGWINCH, sa).ValueOrDie();
+    TEST_PCHECK(sigaction(SIGWINCH, &sa, nullptr) == 0);
+    MaybeSave();
 
     // Indicate to the parent that we're ready.
     write_fd.reset();