perf symbols: Support multiple symtabs in struct thread

Making the routines that were so far specific to the kernel maps
useful for all threads.

This is done by making the kernel maps be contained in a kernel
"thread".

This gets the kernel specific routines closer to the userspace
counterparts, which will help in reducing the boilerplate for
resolving a symbol, as will be demonstrated in the next patches.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1259346563-12568-9-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>