objtool: Fix X86_FEATURE_SMAP alternative handling

objtool: Fix X86_FEATURE_SMAP alternative handling

For X86_FEATURE_SMAP alternatives which replace NOP with STAC or CLAC,
uaccess validation skips the NOP branch to avoid following impossible
code paths, e.g. where a STAC would be patched but a CLAC wouldn't.

However, it's not safe to assume an X86_FEATURE_SMAP alternative is
patching STAC/CLAC.  There can be other alternatives, like
static_cpu_has(), where both branches need to be validated.

Fix that by repurposing ANNOTATE_IGNORE_ALTERNATIVE for skipping either
original instructions or new ones.  This is a more generic approach
which enables the removal of the feature checking hacks and the
insn->ignore bit.

Fixes the following warnings:

  arch/x86/mm/fault.o: warning: objtool: do_user_addr_fault+0x8ec: __stack_chk_fail() missing __noreturn in .c/.h or NORETURN() in noreturns.h
  arch/x86/mm/fault.o: warning: objtool: do_user_addr_fault+0x8f1: unreachable instruction

[ mingo: Fix up conflicts with recent x86 changes. ]

Fixes: ea24213d8088 ("objtool: Add UACCESS validation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/de0621ca242130156a55d5d74fed86994dfa4c9c.1742852846.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/oe-kbuild-all/202503181736.zkZUBv4N-lkp@intel.com/