x86: 32 bit: interrupt stub consistency with 64 bit

Don't generate interrupt stubs for interrupt vectors below
FIRST_EXTERNAL_VECTOR, and make the table of interrupt vectors
(interrupt[]) __initconst.  Both of these changes both conserve memory
and improve consistency with 64 bits.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/arch/x86/include/asm/hw_irq.h b/arch/x86/include/asm/hw_irq.h
index b97aecb..27d33f9 100644
--- a/arch/x86/include/asm/hw_irq.h
+++ b/arch/x86/include/asm/hw_irq.h
@@ -110,7 +110,7 @@
 #endif
 
 #ifdef CONFIG_X86_32
-extern void (*const interrupt[NR_VECTORS])(void);
+extern void (*__initconst interrupt[NR_VECTORS-FIRST_EXTERNAL_VECTOR])(void);
 #endif
 
 typedef int vector_irq_t[NR_VECTORS];