x86/alternatives: Add nested alternatives macros

Instead of making increasingly complicated ALTERNATIVE_n()
implementations, use a nested alternative expression.

The only difference between:

  ALTERNATIVE_2(oldinst, newinst1, flag1, newinst2, flag2)

and

  ALTERNATIVE(ALTERNATIVE(oldinst, newinst1, flag1),
              newinst2, flag2)

is that the outer alternative can add additional padding when the inner
alternative is the shorter one, which then results in
alt_instr::instrlen being inconsistent.

However, this is easily remedied since the alt_instr entries will be
consecutive and it is trivial to compute the max(alt_instr::instrlen) at
runtime while patching.

Specifically, after this the ALTERNATIVE_2 macro, after CPP expansion
(and manual layout), looks like this:

  .macro ALTERNATIVE_2 oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2
  740:
  740: \oldinstr ;
  741: .skip -(((744f-743f)-(741b-740b)) > 0) * ((744f-743f)-(741b-740b)),0x90 ;
  742: .pushsection .altinstructions,"a" ;
  	altinstr_entry 740b,743f,\ft_flags1,742b-740b,744f-743f ;
  .popsection ;
  .pushsection .altinstr_replacement,"ax" ;
  743: \newinstr1 ;
  744: .popsection ; ;
  741: .skip -(((744f-743f)-(741b-740b)) > 0) * ((744f-743f)-(741b-740b)),0x90 ;
  742: .pushsection .altinstructions,"a" ;
  altinstr_entry 740b,743f,\ft_flags2,742b-740b,744f-743f ;
  .popsection ;
  .pushsection .altinstr_replacement,"ax" ;
  743: \newinstr2 ;
  744: .popsection ;
  .endm

The only label that is ambiguous is 740, however they all reference the
same spot, so that doesn't matter.

NOTE: obviously only @oldinstr may be an alternative; making @newinstr
an alternative would mean patching .altinstr_replacement which very
likely isn't what is intended, also the labels will be confused in that
case.

  [ bp: Debug an issue where it would match the wrong two insns and
    and consider them nested due to the same signed offsets in the
    .alternative section and use instr_va() to compare the full virtual
    addresses instead.

    - Use new labels to denote that the new, nested
    alternatives are being used when staring at preprocessed output.

    - Use the %c constraint everywhere instead of %P and document the
      difference for future reference. ]

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Co-developed-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230628104952.GA2439977@hirez.programming.kicks-ass.net

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