x86: Undo return-thunk damage

x86: Undo return-thunk damage

commit 15e67227c49a57837108acfe1c80570e1bd9f962 upstream.

Introduce X86_FEATURE_RETHUNK for those afflicted with needing this.

  [ bp: Do only INT3 padding - simpler. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
[cascardo: CONFIG_STACK_VALIDATION vs CONFIG_OBJTOOL]
[cascardo: no IBT support]
CVE-2022-29900
CVE-2022-29901
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>