x86/alternatives: Add macro comments

... so that when one stares at the .s output, one can find her way
around the resulting asm magic.

With it, ALTERNATIVE looks like this now:

          # ALT: oldnstr
  661:
          ...
  662:
          # ALT: padding
  .skip ...
  663:
  .pushsection .altinstructions,"a"

  ...

  .popsection
  .pushsection .altinstr_replacement, "ax"
          # ALT: replacement 1
  6641:
  	...
  6651:
          .popsection

Merge __OLDINSTR() into OLDINSTR(), while at it.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: X86 ML <x86@kernel.org>
Link: https://lkml.kernel.org/r/20181211222326.14581-2-bp@alien8.de