perf tools: elf_sym__is_function() should accept "zero" sized functions

Asm routines that end up having size equal to zero are not really
zero sized, and as now we do kernel_maps__fixup_sym_end, at least
for kernel routines this gets fixed.

A similar fixup needs to be done for the userspace bits as well,
but as this fixup started only because in /proc/kallsyms we don't
have the end address nor the function size, it appeared here first.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <1254796503-27203-1-git-send-email-acme@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>