x86/bugs: Fix BHI handling of RRSBA

The ARCH_CAP_RRSBA check isn't correct: RRSBA may have already been
disabled by the Spectre v2 mitigation (or can otherwise be disabled by
the BHI mitigation itself if needed).  In that case retpolines are fine.

Fixes: ec9404e4 ("x86/bhi: Add BHI mitigation knob")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/6f56f13da34a0834b69163467449be7f58f253dc.1712813475.git.jpoimboe@kernel.org