Fix gdbserver qGetTLSAddr for x86_64 -m32

gdbserver makes libthread_db to access uninitialized memory.  Surprisingly it
does not harm normally, even -fsanitize=address works with current gdbserver.
I have found just valgrind detects it as a very first warning for gdbserver:

Syscall param ptrace(addr) contains uninitialised byte(s)
   at 0x3721EECEBE: ptrace (ptrace.c:45)
   by 0x436EE5: ps_get_thread_area (linux-x86-low.c:252)
   by 0x5559D02: __td_ta_lookup_th_unique (td_ta_map_lwp2thr.c:157)
   by 0x5559EC3: td_ta_map_lwp2thr (td_ta_map_lwp2thr.c:207)
   by 0x43F87D: find_one_thread (thread-db.c:281)
   by 0x440038: thread_db_get_tls_address (thread-db.c:505)
   by 0x40F6D0: handle_query (server.c:2004)
   by 0x4124CF: process_serial_event (server.c:3445)
   by 0x4136B6: handle_serial_event (server.c:3889)
   by 0x419571: handle_file_event (event-loop.c:434)
   by 0x418D38: process_event (event-loop.c:189)
   by 0x419AB7: start_event_loop (event-loop.c:552)

Reproducible with:
cd gdb/testsuite
g++ -o gdb.threads/tls gdb.threads/tls{,2}.c -m32 -pthread
../gdbserver/gdbserver :1234 gdb.threads/tls
../gdb -batch gdb.threads/tls -ex 'target remote :1234' -ex 'b spin' -ex c -ex 'p a_thread_local'

It is more easily reproducible even without valgrind using s/0x00/0xff/ in the
attached patch.  It will then turn the output of reproducer above:
$1 = 0
->
Cannot find thread-local storage for Thread 29044, executable file .../gdb/testsuite/gdb.threads/tls:
Remote target failed to process qGetTLSAddr request

gdb/gdbserver/
2014-05-19  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Fix gdbserver qGetTLSAddr for x86_64 -m32.
	* linux-x86-low.c (X86_64_USER_REGS): New.
	(x86_fill_gregset): Call memset for BUF first in x86_64 -m32 case.

Message-ID: <20140410114901.GA16411@host2.jankratochvil.net>