x86: Use -mindirect-branch-cs-prefix for RETPOLINE builds

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2090231

commit 68cf4f2a
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Fri, 19 Nov 2021 17:50:25 +0100

    x86: Use -mindirect-branch-cs-prefix for RETPOLINE builds

    In order to further enable commit:

      bbe2df3f ("x86/alternative: Try inline spectre_v2=retpoline,amd")

    add the new GCC flag -mindirect-branch-cs-prefix:

      https://gcc.gnu.org/g:2196a681d7810ad8b227bf983f38ba716620545e
      https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102952
      https://bugs.llvm.org/show_bug.cgi?id=52323



    to RETPOLINE=y builds. This should allow fully inlining retpoline,amd
    for GCC builds.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
    Link: https://lkml.kernel.org/r/20211119165630.276205624@infradead.org



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