x86: unify include/asm-x86/linkage_[32|64].h

Remove definitions of FASTCALL/fastcall from linkage_32 as compiled with
-regparm=3 by default since 2.6.20 and should no longer be needed.

CONFIG X86_64 and CONFIG_X86_ALIGNMENT_16 are mutually exclusive as found
in Kconfig.cpu so it should be fine to test them separately.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>