x86/alternative: Handle Jcc __x86_indirect_thunk_\reg

Handle the rare cases where the compiler (clang) does an indirect
conditional tail-call using:

  Jcc __x86_indirect_thunk_\reg

For the !RETPOLINE case this can be rewritten to fit the original (6
byte) instruction like:

  Jncc.d8	1f
  JMP		*%\reg
  NOP
1:

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.296470217@infradead.org