x86: common.c boot_cpu_stack and boot_exception_stacks should be static

Impact: cleanup, avoid sparse warnings, reduce kernel size a bit

Fixes these sparse warnings:

 arch/x86/kernel/cpu/common.c:869:6: warning: symbol 'boot_cpu_stack' was not declared. Should it be static?
 arch/x86/kernel/cpu/common.c:910:6: warning: symbol 'boot_exception_stacks' was not declared. Should it be static?

Signed-off-by: Jaswinder Singh <jaswinder@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>