use gettid on Linux after all, via syscall()

src/runtime/process.cpp

@@ -27,6 +27,10 @@ Author: Jared Roesch
 #include <limits.h> // NOLINT
 #endif
 
+#ifdef __linux
+#include <sys/syscall.h>
+#endif
+
 #include "runtime/object.h"
 #include "runtime/io.h"
 #include "runtime/array_ref.h"
@@ -323,8 +327,9 @@ extern "C" LEAN_EXPORT obj_res lean_io_get_thread_id(obj_arg) {
 #elif defined(__APPLE__)
     pthread_threadid_np(NULL, &tid);
 #else
-    // should use gettid, but that is glibc 2.30+
-    tid = static_cast<uint64>(pthread_self());
+    // since Linux 2.4.11, our glibc 2.27 requires at least 3.2
+    // glibc 2.30 would provide a wrapper
+    tid = (pid_t)syscall(SYS_gettid);
 #endif
     return lean_io_result_mk_ok(box_uint64(tid));
 }