x86_64: Check for .cfi_rel_offset in CFI probe

Very old 64bit binutils have .cfi_startproc/endproc, but
no .cfi_rel_offset. Check for .cfi_rel_offset too.

Cc: Jan Beulich <jbeulich@novell.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>