x86: change FIRST_SYSTEM_VECTOR to a variable, fix

Fixes the build error introduced by my FIRST_SYSTEM_VECTOR patch

Signed-off-by: Alan Mayer <ajm@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>