target-i386: Fix signedness of MSR_IA32_APICBASE_BASE

Existing definition triggers the following when using clang
-fsanitize=undefined:

    hw/intc/apic_common.c:314:55: runtime error: left shift of 1048575 by 12
        places cannot be represented in type 'int'

Fix it so we won't try to shift a 1 to the sign bit of a signed integer.
Suggested-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

target-i386/cpu.h

@@ -305,7 +305,7 @@
 #define MSR_IA32_APICBASE               0x1b
 #define MSR_IA32_APICBASE_BSP           (1<<8)
 #define MSR_IA32_APICBASE_ENABLE        (1<<11)
-#define MSR_IA32_APICBASE_BASE          (0xfffff<<12)
+#define MSR_IA32_APICBASE_BASE          (0xfffffU<<12)
 #define MSR_IA32_FEATURE_CONTROL        0x0000003a
 #define MSR_TSC_ADJUST                  0x0000003b
 #define MSR_IA32_TSCDEADLINE            0x6e0