x86/speculation: Make all RETbleed mitigations depend on X86_64

x86/speculation: Make all RETbleed mitigations depend on X86_64

Forwarded: https://lore.kernel.org/lkml/YtwSR3NNsWp1ohfV@decadent.org.uk/T/

The mitigations for RETBleed are currently ineffective on x86_32 since
entry_32.S does not use the required macros.  However, for an x86_32
target, the kconfig symbols for them are still enabled by default and
/sys/devices/system/cpu/vulnerabilities/retbleed will wrongly report
that mitigations are in place.

Make all of these symbols depend on X86_64, and only enable RETHUNK by
default on X86_64.

Cc: stable@vger.kernel.org
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/x86
Gbp-Pq: Name x86-speculation-make-all-retbleed-mitigations-depend.patch