x86/cpu: Fix up srso_safe_ret() and __x86_return_thunk()

  vmlinux.o: warning: objtool: srso_untrain_ret() falls through to next function __x86_return_skl()
  vmlinux.o: warning: objtool: __x86_return_thunk() falls through to next function __x86_return_skl()

This is because these functions (can) end with CALL, which objtool
does not consider a terminating instruction. Therefore, replace the
INT3 instruction (which is a non-fatal trap) with UD2 (which is a
fatal-trap).

This indicates execution will not continue past this point.

Fixes: fb3bd914 ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230814121148.637802730@infradead.org