[ARM] Shut up warning about init_thread_union

Fix false warning:

WARNING: arch/arm/kernel/init_task.o - Section mismatch:
 reference to .init.task:init_thread_union from .data between
 'init_task' (at offset 0x4) and 'init_sighand'

caused by the section name starting with ".init".

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>