Cygwin: raise: change to call pthread_kill

POSIX requires that raise(3) is equivalent to

  pthread_kill(pthread_self(), sig);

in multi-threaded applications.  Our raise just called kill(sig).
Fix that.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/release/3.0

@@ -89,3 +89,6 @@ Bug Fixes
 
 - Disallow seteuid on disabled or locked out accounts.
   Addresses: https://cygwin.com/ml/cygwin/2019-01/msg00197.html
+
+- Fix raise to work as required by POSIX.
+  (Partially) addresses: https://cygwin.com/ml/cygwin/2019-01/msg00149.html

winsup/cygwin/signal.cc

@@ -300,7 +300,10 @@ _pinfo::kill (siginfo_t& si)
 extern "C" int
 raise (int sig)
 {
-  return kill (myself->pid, sig);
+  pthread *thread = _my_tls.tid;
+  if (!thread)
+    return kill (myself->pid, sig);
+  return pthread_kill (thread, sig);
 }
 
 static int