arm: unify interrupt init

all arm init the IRQ stack the same way
so unify it in lib_arm/interrupts.c and then call arch specific interrupt init

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/cpu/ixp/interrupts.c b/cpu/ixp/interrupts.c
index a05e439..06a826a 100644
--- a/cpu/ixp/interrupts.c
+++ b/cpu/ixp/interrupts.c
@@ -67,7 +67,7 @@
 	IRQ_HANDLER[irq].m_func = handle_irq;
 }
 
-int interrupt_init (void)
+int arch_interrupt_init (void)
 {
 	int i;