objtool: Fix retpoline support for pre-ORC objtool

Objtool 1.0 (pre-ORC) produces the following warning when it encounters
a retpoline:

  arch/x86/crypto/camellia-aesni-avx2-asm_64.o: warning: objtool: .altinstr_replacement+0xf: return instruction outside of a callable function

That warning is meant to catch GCC bugs and missing ENTRY/ENDPROC
annotations, neither of which are applicable to alternatives.  Silence
the warning for alternative instructions, just like objtool 2.0 already
does.

Reported-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>