x86: head_64.S cleanup - use predefined flags from processor-flags.h

We should better use already defined flags from processor-flags.h instead
of defining own ones

[>>> object code check >>>]

original
md5sum: 9cfa6dbf045a046bb5dfb85f8bcfe8c4  arch/x86/kernel/head_64.o
   text    data     bss     dec     hex filename
  37361    4432    8192   49985    c341 arch/x86/kernel/head_64.o

patched
md5sum: 9cfa6dbf045a046bb5dfb85f8bcfe8c4  arch/x86/kernel/head_64.o
   text    data     bss     dec     hex filename
  37361    4432    8192   49985    c341 arch/x86/kernel/head_64.o

[<<< object code check <<<]

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>