gdb.base/sigstep.exp: xfail gdb/17511 on i?86 Linux

gdb.base/sigstep.exp: xfail gdb/17511 on i?86 Linux

Running gdb.base/sigstep.exp with --target=i686-pc-linux-gnu on a
64-bit kernel naturally trips on PR gdb/17511 as well, given this is a
kernel bug.

I haven't really tested a real 32-bit kernel/machine, but given the
code in question in the kernel is shared between 32-bit and 64-bit,
I'm quite sure the bug triggers in those cases as well.

So, simply xfail i?86-*-linux* too.

gdb/testsuite/
2014-11-07  Pedro Alves  <palves@redhat.com>

	PR gdb/17511
	* gdb.base/sigstep.exp (in_handler_map) <si+advance>: xfail
	i?86-*-linux*.