fix gcc4 compiler error on sgi

[Marco Peereboom]

This is just like the mips64 code.

Index: arcbios.c
===================================================================
RCS file: /cvs/src/sys/arch/sgi/stand/boot/arcbios.c,v
retrieving revision 1.12
diff -u -p -r1.12 arcbios.c
--- arcbios.c   22 Jul 2009 20:23:44 -0000      1.12
+++ arcbios.c   14 Sep 2010 02:23:45 -0000
@@ -36,7 +36,7 @@
 
 #include <stand.h>
 
-static int     bios_is_32bit;
+int    bios_is_32bit;
 
 u_int  kl_n_shift = 32;
 int    arcbios_init(void);