[CORE] vmm_percpu: Make sure __percpu_offset[] table is zero initialized

We use __percpu_offset[] table which hold relative offset of percpu section
for each host CPU. The this_cpu() and per_cpu() macros depend on this
__percpu_offset[] table for getting percpu variables.

It is quite possible the __percpu_offset[] table is accessed even before
vmm_percpu_init() hence this patch ensures that it is zero initialized
at boot-time by moving it to .bss section.

Tested-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
Signed-off-by: Anup Patel <anup@brainfault.org>