Add helper script for glibc debugging

Add helper script for glibc debugging

This patch adds a new make rule that generates a helper script for
debugging glibc test cases.  The new script, debugglibc.sh, is similar
to testrun.sh, in the sense that it allows the execution of the
specified test case, however, it opens the test case in GDB, setting the
library path the same way that testrun.sh does.  The commands are based
on the instructions on the wiki for glibc debugging [1,2].

By default, the script tells GDB to load the test case for symbol
information, so that, when a breakpoint is hit, the call stack is
displayed correctly (instead of printing lots of '??'s).  For instance,
after running 'make' and 'make check', one could do the following:

  $ ./debugglibc.sh nptl/tst-exec1 -b pthread_join

  Reading symbols from /home/gabriel/build/powerpc64le/glibc//elf/ld.so...done.
  Breakpoint 1 at 0x1444
  add symbol table from file "nptl/tst-exec1"
  [Thread debugging using libthread_db enabled]
  Using host libthread_db library "/home/gabriel/build/powerpc64le/glibc//nptl_db/libthread_db.so.1".

  Breakpoint 1, 0x00007ffff7fb1444 in _dl_start_user () from /home/gabriel/build/powerpc64le/glibc/elf/ld.so
  Breakpoint 2 at 0x7ffff7f49d48: file pthread_join.c, line 23.

Notice that the script will always start GDB with the program running
and halted at _dl_start_user.  So, in order to reach the actual
breakpoint of interest, one should hit 'c', not 'r':

  >>> c
  Continuing.
  [New Thread 0x7ffff7d1f180 (LWP 76443)]
  [Switching to Thread 0x7ffff7d1f180 (LWP 76443)]

  Thread 2 "ld.so" hit Breakpoint 2, __pthread_join (threadid=140737354087616, thread_return=0x0) at pthread_join.c:24
  24        return __pthread_timedjoin_ex (threadid, thread_return, NULL, true);

Then inspect the call stack with 'bt', as usual, and see symbols from
both the test case and from the libraries themselves:

  >>> bt
  #0  __pthread_join (threadid=140737354087616, thread_return=0x0) at pthread_join.c:24
  #1  0x0000000010001f4c in tf (arg=<optimized out>) at tst-exec1.c:37
  #2  0x00007ffff7f487e8 in start_thread (arg=0x7ffff7510000) at pthread_create.c:479
  #3  0x00007ffff7e523a8 in clone () at ../sysdeps/unix/sysv/linux/powerpc/powerpc64/clone.S:82

Tested for powerpc64le and x86_64.

[1] https://sourceware.org/glibc/wiki/Debugging/Loader_Debugging
[2] https://sourceware.org/glibc/wiki/Testing/Builds#Required_gdb_setup

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Dmitry V. Levin <ldv@altlinux.org>
Reviewed-by: Joseph Myers <joseph@codesourcery.com>
Reviewed-by: Andreas Schwab <schwab@suse.de>