[gdb/testsuite] Add gdb.opt/break-on-_exit.exp

Add a test-case to excercise the problem scenario reported in PR28527 and
fixed in commit a50bdb99af "[gdb/tdep, rs6000] Don't skip system call in
skip_prologue":
- set a breakpoint on _exit, and
- verify that it triggers.

Note that this is not a regression test for that commit.  Since the actual
code in _exit may vary across os instances, we cannot guarantee that the
problem will always trigger with this test-case.

Rather, this test-case is a version of the original test-case
(gdb.threads/process-dies-while-detaching.exp) that is minimal while still
reproducing the problem reported in PR28527, in that same setting.

The benefit of this test-case is that it exercise real-life code and may
expose similar problems in other settings.  Also, it provides a much easier
test-case to investigate in case a similar problem occurs.

Tested on x86_64-linux and ppc64le-linux.

gdb/testsuite/gdb.opt/break-on-_exit.c

@@ -0,0 +1,26 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2021 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+
+int
+main (void)
+{
+  _exit (0);
+
+  return 0;
+}

gdb/testsuite/gdb.opt/break-on-_exit.exp

@@ -0,0 +1,66 @@
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program 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 General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+# Set a breakpoint on libc's _exit, and verify that it triggers.  The function
+# tends to do a syscall immediately after the prologue, and if the breakpoint is
+# set past the syscall due to faulty prologue skipping, the breakpoint will not
+# trigger.
+#
+# In particular, we're trying to excercise the instruction analysis
+# functionality of prologue skipping.  If the non-minimal symbols are
+# read for libc, then that functionality might not be used because f.i.
+# line-info is used instead.  Also, if the minimal symbols are not read
+# for libc, then the breakpoint is set on the exec-local _exit@plt instead,
+# and that functionality will also not be used.
+#
+# We may get the required setup in case of a libc with misssing separate
+# debuginfo, but we want the same effect if that debuginfo is installed.
+#
+# So, we use -readnever to read minimal symbols, but not non-miminal symbols.
+#
+# Because the code at _exit may be and usually is optimized, the test is in
+# the gdb.opt directory.
+
+standard_testfile
+
+# See if we have target board readnow.exp or similar.
+if { [lsearch -exact $GDBFLAGS -readnow] != -1 \
+	 || [lsearch -exact $GDBFLAGS --readnow] != -1 } {
+    untested "--readnever not allowed in combination with --readnow"
+    return -1
+}
+
+save_vars { GDBFLAGS } {
+    append GDBFLAGS " -readnever"
+
+    if {[prepare_for_testing "failed to prepare" $testfile $srcfile nodebug]} {
+	return -1
+    }
+}
+
+if ![runto_main] then {
+    return 0
+}
+
+gdb_breakpoint "_exit"
+
+# Give some background information about the breakpoint(s) and corresponding
+# the shared lib(s).
+gdb_test "info breakpoints"
+gdb_test "info shared"
+
+# If the skip_prologue analysis of _exit is too eager, we may not hit the
+# breakpoint.
+gdb_continue_to_breakpoint "_exit" "_exit \\(\\) .*"