Add some cfi directives in the code doing syscall (by Valgrind).

This allows to attach to Valgrind when VAlgrind is blocked in a syscall
and have GDB producing a stacktrace, rather than being unable
to unwind.
I.e. instead of having:
  (gdb) bt
  #0  0x380460f2 in do_syscall_WRK ()
  (gdb) 
with the directives, we obtain:
   (gdb) bt
   #0  vgPlain_mk_SysRes_x86_linux (val=1) at m_syscall.c:65
   svn2github#1  vgPlain_do_syscall (sysno=168, a1=944907996, a2=1, a3=4294967295, a4=0, a5=0, a6=0, a7=0, a8=0) at m_syscall.c:791
   svn2github#2  0x38031986 in vgPlain_poll (fds=0x385226dc <remote_desc_pollfdread_activity>, nfds=1, timeout=-1) at m_libcfile.c:535
   svn2github#3  0x3807479f in vgPlain_poll_no_eintr (fds=0x385226dc <remote_desc_pollfdread_activity>, nfds=1, timeout=-1)
       at m_gdbserver/remote-utils.c:86
   svn2github#4  0x380752f0 in readchar (single=4096) at m_gdbserver/remote-utils.c:938
   svn2github#5  0x38075ae3 in getpkt (buf=0x61f35020 "") at m_gdbserver/remote-utils.c:997
   svn2github#6  0x38076fcb in server_main () at m_gdbserver/server.c:1048
   svn2github#7  0x38072af2 in call_gdbserver (tid=1, reason=init_reason) at m_gdbserver/m_gdbserver.c:721
   #8  0x380735ba in vgPlain_gdbserver (tid=1) at m_gdbserver/m_gdbserver.c:788
   #9  0x3802c6ef in do_actions_on_error (allow_db_attach=<optimized out>, err=<optimized out>) at m_errormgr.c:532
   #10 pp_Error (err=0x61f580e0, allow_db_attach=1 '\001', xml=1 '\001') at m_errormgr.c:644
   #11 0x3802cc34 in vgPlain_maybe_record_error (tid=1643479264, ekind=8, a=2271560481, s=0x0, extra=0x62937f1c)
       at m_errormgr.c:851
   #12 0x38028821 in vgMemCheck_record_free_error (tid=1, a=2271560481) at mc_errors.c:836
   #13 0x38007b65 in vgMemCheck_free (tid=1, p=0x87654321) at mc_malloc_wrappers.c:496
   #14 0x3807e261 in do_client_request (tid=1) at m_scheduler/scheduler.c:1840
   #15 vgPlain_scheduler (tid=1) at m_scheduler/scheduler.c:1406
   #16 0x3808b6b2 in thread_wrapper (tidW=<optimized out>) at m_syswrap/syswrap-linux.c:102
   #17 run_a_thread_NORETURN (tidW=1) at m_syswrap/syswrap-linux.c:155
   #18 0x00000000 in ?? ()
   (gdb) 




git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15194 a5019735-40e9-0310-863c-91ae7b9d1cf9

coregrind/m_syscall.c

@@ -305,10 +305,15 @@ asm(
 ".text\n"
 ".globl do_syscall_WRK\n"
 "do_syscall_WRK:\n"
+"	.cfi_startproc\n"
 "	push	%esi\n"
+"	.cfi_adjust_cfa_offset 4\n"
 "	push	%edi\n"
+"	.cfi_adjust_cfa_offset 4\n"
 "	push	%ebx\n"
+"	.cfi_adjust_cfa_offset 4\n"
 "	push	%ebp\n"
+"	.cfi_adjust_cfa_offset 4\n"
 "	movl	16+ 4(%esp),%eax\n"
 "	movl	16+ 8(%esp),%ebx\n"
 "	movl	16+12(%esp),%ecx\n"
@@ -318,10 +323,15 @@ asm(
 "	movl	16+28(%esp),%ebp\n"
 "	int	$0x80\n"
 "	popl	%ebp\n"
+"	.cfi_adjust_cfa_offset -4\n"
 "	popl	%ebx\n"
+"	.cfi_adjust_cfa_offset -4\n"
 "	popl	%edi\n"
+"	.cfi_adjust_cfa_offset -4\n"
 "	popl	%esi\n"
+"	.cfi_adjust_cfa_offset -4\n"
 "	ret\n"
+"	.cfi_endproc\n"
 ".previous\n"
 );