xen/x86: only support >128 CPUs on x86_64

32 bit cannot cope with 256 cpus and hits:

    /* At least half the ioremap space should be available to us. */
    BUILD_BUG_ON(IOREMAP_VIRT_START + (IOREMAP_MBYTES << 19) >=
    FIXADDR_START);

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>

xen/include/asm-x86/config.h

@@ -49,6 +49,8 @@
 
 #ifdef MAX_PHYS_CPUS
 #define NR_CPUS MAX_PHYS_CPUS
+#elif defined __i386__
+#define NR_CPUS 128
 #else
 #define NR_CPUS 256
 #endif