LinuxSyscalls: Fixes exit syscall

if an application is using `exit` then it is usually a faulting
condition rather than cleanly exiting. When cleanly exiting
applications will typically use `exit_group` instead.

`exit` is useful to quickly cause a single thread to exit in a
multi-threaded environment as well, where `exit_group` will take down
the entire process group.

FEX had implemented this in a way that would do a double Stop signal,
cascading to a crash. When tied in to a crash handler, this could get
caught in a weird way.

This /should/ fix #4198, but I can't confirm locally. It looks like in
that issue that the steam install is slightly buggered (as evident by
missing srt-logger and steam-runtime-identify-library-abi).

This is a bug regardless so fix it and create a unittest. If it doesn't
fix the user's bug, then we have another workaround that will definitely
solve it.

Source/Tools/LinuxEmulation/LinuxSyscalls/Syscalls/Thread.cpp

@@ -414,9 +414,10 @@ void RegisterThread(FEX::HLE::SyscallHandler* Handler) {
                                 }
 
                                 ThreadObject->StatusCode = status;
-                                FEX::HLE::_SyscallHandler->TM.StopThread(ThreadObject);
-
-                                return 0;
+                                FEX::HLE::_SyscallHandler->TM.DestroyThread(ThreadObject, true);
+                                syscall(SYSCALL_DEF(exit), status);
+                                // This will never be reached
+                                std::terminate();
                               });
 
   REGISTER_SYSCALL_IMPL_FLAGS(prctl, SyscallFlags::OPTIMIZETHROUGH | SyscallFlags::NOSYNCSTATEONENTRY,

unittests/FEXLinuxTests/tests/syscalls/syscall_exit.cpp

@@ -0,0 +1,35 @@
+#include <catch2/catch_test_macros.hpp>
+
+#include <unistd.h>
+#include <sys/wait.h>
+#include <sys/syscall.h>
+
+TEST_CASE("fork - exit") {
+  int child_pid = ::fork();
+  if (child_pid == 0) {
+    ::syscall(SYS_exit, 1);
+    // unreachable
+    std::terminate();
+  } else {
+    int status {};
+    int exited_child = ::waitpid(child_pid, &status, 0);
+    bool exited = WIFEXITED(status);
+    REQUIRE(WIFEXITED(status) == 1);
+    CHECK(WEXITSTATUS(status) == 1);
+  }
+}
+
+TEST_CASE("fork - signal") {
+  int child_pid = ::fork();
+  if (child_pid == 0) {
+    ::syscall(SYS_tgkill, ::getpid(), ::gettid(), SIGKILL);
+    // unreachable
+    std::terminate();
+  } else {
+    int status {};
+    int exited_child = ::waitpid(child_pid, &status, 0);
+    bool exited = WIFEXITED(status);
+    REQUIRE(WIFSIGNALED(status) == 1);
+    CHECK(WTERMSIG(status) == SIGKILL);
+  }
+}