x86: Add straight-line-speculation mitigation

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2090231
Conflicts: Merge conflict in arch/x86/lib/retpoline.S due to the
	   presence of a later upstream commit d45476d9
	   ("x86/speculation: Rename RETPOLINE_AMD to RETPOLINE_LFENCE").

commit e463a09a
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Sat, 4 Dec 2021 14:43:44 +0100

    x86: Add straight-line-speculation mitigation

    Make use of an upcoming GCC feature to mitigate
    straight-line-speculation for x86:

      https://gcc.gnu.org/g:53a643f8568067d7700a9f2facc8ba39974973d3
      https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102952
      https://bugs.llvm.org/show_bug.cgi?id=52323



    It's built tested on x86_64-allyesconfig using GCC-12 and GCC-11.

    Maintenance overhead of this should be fairly low due to objtool
    validation.

    Size overhead of all these additional int3 instructions comes to:

         text          data     bss     dec     hex filename
      22267751      6933356 2011368 31212475        1dc43bb defconfig-build/vmlinux
      22804126      6933356 1470696 31208178        1dc32f2 defconfig-build/vmlinux.sls

    Or roughly 2.4% additional text.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
    Link: https://lore.kernel.org/r/20211204134908.140103474@infradead.org



Signed-off-by: Waiman Long <longman@redhat.com>