[Backtracing][Linux] Include declarations for gettid() and tgkill().

Older glibc doesn't include these.  Nor does Musl, it seems.  The
previous fix put the gettid() declaration too far down the file.

rdar://110417355

stdlib/public/runtime/CrashHandlerLinux.cpp

@@ -183,6 +183,18 @@ _swift_installCrashHandler()
 
 namespace {
 
+// Older glibc and musl don't have these two syscalls
+pid_t
+gettid()
+{
+  return (pid_t)syscall(SYS_gettid);
+}
+
+int
+tgkill(int tgid, int tid, int sig) {
+  return syscall(SYS_tgkill, tgid, tid, sig);
+}
+
 void
 reset_signal(int signum)
 {
@@ -296,12 +308,6 @@ getdents(int fd, void *buf, size_t bufsiz)
   return syscall(SYS_getdents64, fd, buf, bufsiz);
 }
 
-pid_t
-gettid()
-{
-  return (pid_t)syscall(SYS_gettid);
-}
-
 /* Stop all other threads in this process; we do this by establishing a
    signal handler for SIGPROF, then iterating through the threads sending
    SIGPROF.