handle an unspecified return address column

handle an unspecified return address column

Debugging PR 16155 further, I found that the DWARF unwinder found the
function in question, but thought it had no registers saved
(fs->regs.num_regs == 0).

It seems to me that if a frame does not specify the return address
column, or if the return address column is explicitly marked as
DWARF2_FRAME_REG_UNSPECIFIED, then we should set the
"undefined_retaddr" flag and let the DWARF unwinder gracefully stop.

This patch implements that idea.

With this patch the backtrace works properly:

    (gdb) bt
    #0  0x0000007fb7ed485c in nanosleep () from /lib64/libc.so.6
    #1  0x0000007fb7ed4508 in sleep () from /lib64/libc.so.6
    #2  0x00000000004008bc in thread_function (arg=0x4) at threadapply.c:73
    #3  0x0000007fb7fad950 in start_thread () from /lib64/libpthread.so.0
    #4  0x0000007fb7f0956c in clone () from /lib64/libc.so.6

2013-11-22  Tom Tromey  <tromey@redhat.com>

	PR backtrace/16155:
	* dwarf2-frame.c (dwarf2_frame_cache): Set undefined_retaddr if
	the return address column is unspecified.

2013-11-22  Tom Tromey  <tromey@redhat.com>

	* gdb.dwarf2/dw2-bad-cfi.c: New file.
	* gdb.dwarf2/dw2-bad-cfi.exp: New file.
	* gdb.dwarf2/dw2-bad-cfi.S: New file.