Bug 18125: Call exit after last linked context.

There appears to be a discrepancy among the implementations
of setcontext with regards to the function called once the last
linked-to context has finished executing via setcontext.

The POSIX standard says:
~~~
If the uc_link member of the ucontext_t structure pointed to by
the ucp argument is equal to 0, then this context is the main
context, and the thread will exit when this context returns.
~~~

It says "exit" not "exit immediately" nor "exit without running
functions registered with atexit or on_exit."

Therefore the AArch64, ARM, hppa and NIOS II implementations are
wrong and no test detects it.

It is questionable if this should even be fixed or just documented
that the above 4 targets are wrong. The functions are deprecated
and nobody should be using them, but at the same time it silly to
have cross-target differences that make it hard to port old
applications from say x86_64 to AArch64.

Therefore I will ix the 4 arches, and checkin a regression
test to prevent it from changing again.

https://sourceware.org/ml/libc-alpha/2015-03/msg00720.html

stdlib/tst-setcontext3.sh

@@ -0,0 +1,54 @@
+#! /bin/bash
+# Bug 18125: Test the exit functionality of setcontext().
+# Copyright (C) 2015 Free Software Foundation, Inc.
+# This file is part of the GNU C Library.
+
+# The GNU C Library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+
+# The GNU C Library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+
+# You should have received a copy of the GNU Lesser General Public
+# License along with the GNU C Library; if not, see
+# <http://www.gnu.org/licenses/>.
+
+set -e
+
+common_objpfx=$1
+test_program_prefix_before_env=$2
+run_program_env=$3
+test_program_prefix_after_env=$4
+objpfx=$5
+
+test_pre="${test_program_prefix_before_env} ${run_program_env}"
+test="${test_program_prefix_after_env} ${objpfx}tst-setcontext3"
+out=${objpfx}tst-setcontext3.out
+
+tempfiles=()
+cleanup() {
+  rm -f "${tempfiles[@]}"
+}
+trap cleanup 0
+
+tempfile=$(mktemp "tst-setcontext3.XXXXXXXXXX")
+tempfiles+=("$tempfile")
+
+# We want to run the test program and see if secontext called
+# exit() and wrote out the test file we specified.  If the
+# test exits with a non-zero status this will fail because we
+# are using `set -e`.
+$test_pre $test "$tempfile"
+
+# Look for resulting file.
+if [ -e "$tempfile" ]; then
+  echo "PASS: tst-setcontext3 an exit() and created $tempfile"
+  exit 0
+else
+  echo "FAIL: tst-setcontext3 did not create $tempfile"
+  exit 1
+fi