x86/asm: Simplify __smp_mb() definition

Drop the bitness ifdeffery in favor of using _ASM_SP,
which is the helper macro for the rSP register specification
for 32 and 64 bit depending on the build.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210512093310.5635-1-bp@alien8.de